One page says cannabis clears in three days. Another says 30. A forum post may describe a positive result after nearly three months. If you are trying to learn how long does weed stay in your system, those answers conflict because they often involve different specimens, cutoffs, use patterns, and levels of evidence.
The practical answer depends on what is tested. This comparison shows why a controlled result and a personal story should not carry the same weight:
| Specimen | What science supports | What anecdotes report | How to interpret the gap |
|---|---|---|---|
| Urine | A 50 ng/mL screen may detect a single exposure for about 1 to 2 days in controlled research. Repeated use can remain positive for days to weeks, and some frequent users take longer. | Forum posts describe negatives within days and positives after 80 to 90 days. | Use pattern, urine concentration, cutoff, and confirmation rules can change the result. Unverified posts cannot establish abstinence or laboratory conditions. |
| Hair | A proximal 3 cm scalp segment represents roughly three months of growth, with a short lag before new hair can be sampled. Testing is less sensitive to light use. | Posters describe passes at 63 days and failures after 125 days. | Segment length, growth, sample mass, cosmetic treatment, analytes, and laboratory procedures are usually unknown. |
| Saliva | Detection often lasts hours and may extend into the next day or longer. Controlled results change substantially with the analyte combination and cutoff. An unusual chronic-use study found intermittent THC for up to eight days. | Some posters claim a same-day pass; others allow several days. | A reported pass does not validate a general window. Device, cutoff, use history, and laboratory confirmation are rarely documented. |
| Blood | Parent THC commonly falls within hours after a single dose, but sensitive assays can detect low residual concentrations for days in some frequent users. | Personal reports rarely include a verified concentration, collection time, or threshold. | Blood is associated with more recent exposure than urine, but a concentration does not reliably prove impairment or the exact time of use. |
These are evidence-informed ranges, not guaranteed deadlines. Three terms must remain separate:
- Detectable means an instrument measured an analyte, such as THC or THC-COOH, above its analytical limit.
- Positive means the result met a testing program’s reporting criteria. A detectable amount can remain below a positive cutoff.
- Impairment means cannabis is affecting function or behavior. Detection alone does not establish impairment.
Controlled-administration and monitored-abstinence studies provide the strongest estimates because researchers know the dose or supervise abstinence. Anecdotes can illustrate uncertainty, but they cannot predict an individual’s result.
How Long Does a Weed High Last?
Smoked or vaped cannabis usually starts producing noticeable effects within minutes. The main high often lasts about 2 to 4 hours, although dose, potency, tolerance, and individual response can shorten or extend that period. In a controlled crossover trial of infrequent users, some subjective effects and cognitive or psychomotor changes persisted for up to six hours on average after smoked or vaporized cannabis.
Edibles follow a slower curve. Effects may begin approximately 30 to 120 minutes after ingestion, and delayed onset can make the first two hours feel deceptively quiet. In a controlled brownie study, subjective and cognitive effects peaked 1.5 to 3 hours after ingestion and lasted 6 to 8 hours. Higher doses may feel stronger, and individual effects can last outside those study averages.
| Route | Usual onset | Peak or main high | Effects documented in controlled research |
|---|---|---|---|
| Smoked cannabis | Within minutes | Often about 2 to 4 hours | Some subjective and performance effects persisted up to 6 hours |
| Vaped cannabis | Within minutes | Often about 2 to 4 hours | Some subjective and performance effects persisted up to 6 hours |
| Edible cannabis | Approximately 30 to 120 minutes | Peak commonly 1.5 to 3 hours after ingestion | Effects commonly lasted 6 to 8 hours in the cited study |
The duration of the high is not the THC detection window. A person may no longer feel high while urine still contains THC-COOH, blood or saliva contains measurable THC, or a later hair segment records exposure. Feeling sober does not establish a negative test result.
What Science Says
A credible THC detection window must identify five things: the specimen, analyte, cutoff, use pattern, and study design. Remove any one of them and a precise-looking number can become misleading.
How Long Does Weed Stay in Urine?
Urine testing usually looks for delta-9-tetrahydrocannabinol-9-carboxylic acid, also called THC-COOH or THCA in federal testing documents. This inactive breakdown product is different from the parent THC responsible for intoxication. A urine result therefore supports prior exposure, not current impairment.
The cutoff matters as much as the calendar. Under SAMHSA’s 2024 Medical Review Officer guidance for federal workplace programs, the initial marijuana-metabolite screen uses a 50 ng/mL cutoff. A specimen that is not negative on the initial test then requires confirmation of the specific metabolite at 15 ng/mL. Private employers, courts, clinicians, laboratories, and home tests may use different rules.
A small controlled study illustrates the effect of dose and cutoff. Six participants each smoked a placebo, 1.75% THC cigarette, or 3.55% THC cigarette during separate sessions. At a 15 ng/mL gas chromatography-mass spectrometry cutoff, mean last THC-COOH detection was 33.7 hours after the lower-dose cigarette and 88.6 hours after the higher-dose cigarette, according to the 1995 Huestis, Mitchell, and Cone study. At a 50 ng/mL immunoassay cutoff, mean detection was less than one day after the lower dose and less than two days after the higher dose.
Those findings do not establish a universal one-time-use deadline. The sample was small, the cigarettes were much less potent than many modern products, and assay performance differed. Even within the study, detection varied by participant, dose, cutoff, and test.
Repeated use produces a longer and more variable window. In a monitored-abstinence study of 60 cannabis users, researchers analyzed 6,158 urine specimens. Participants were grouped by their initial creatinine-normalized THC-COOH concentration. Mean times to the last positive 50 ng/mL screen were 4.3, 9.7, and 15.4 days in the low, middle, and high groups.
Those averages conceal individual variation. The same study documented negative specimens followed by later positives during monitored abstinence. Near a cutoff, changes in urine concentration and intermittent residual metabolite excretion can move a result across the reporting line without establishing new use.
A later study began with 70 adolescents and young adults during one month of biochemically verified abstinence. Among the 62 participants who completed more than 25 days, 40% still had THC-COOH above a sensitive 5 ng/mL quantitation limit, and 19% met federal-style 50 ng/mL screening and 15 ng/mL confirmation criteria. The modeled detection window was 10 days, with an estimated range of 4 to 80 days.
So, how long does weed stay in urine? After a single controlled exposure, a 50 ng/mL screen often has a narrow window of about one to two days. Lower cutoffs can extend it. Sustained use can remain positive for days to weeks, and some frequent users remain positive longer. Only the actual analyte, cutoff, and confirmation process define a reportable positive.
How Long Does Weed Stay in Hair?
Hair provides a long historical view, but it is not a perfectly dated record. Scalp hair grows at approximately 1 cm per month, so a proximal 3 cm segment is commonly interpreted as about three months of growth.
New hair must emerge far enough above the scalp to be collected. This creates an approximate 7 to 10-day lag for very recent exposure. Growth rates vary, and a laboratory may test a different length, so 90 days is an estimate rather than an exact calendar boundary.
Sensitivity is another limitation. A study recruited 136 participants, but the main analysis included 105 samples that met the minimum hair-mass requirement. THC was detected in 77% of self-reported heavy users and 39% of light users, compared with none of the non-users. The researchers concluded that hair can indicate heavy use over the previous three months but cannot reliably detect light use or determine how much was used in a specific month.
Parent THC can enter hair through blood, sweat, and sebum, the oily substance produced by skin. Environmental contamination can also deposit THC on hair. THC-COOH is produced in the body and is more specific to ingestion, but it occurs in hair at very low concentrations and is difficult to measure.
The Society of Hair Testing consensus recommendations emphasize standardized collection, decontamination, analyte selection, cutoffs, and specimen-specific interpretation. Sample location, hair damage, cosmetic treatment, segment length, and external exposure can all complicate a result.
How long does weed stay in hair? A 3 cm scalp sample often represents roughly 90 days of growth, excluding the short period before newly formed hair can be collected. A negative result cannot reliably exclude light use. A positive result does not establish current impairment or a precise consumption date.
How Long Does Weed Stay in Saliva?
Saliva, also called oral fluid, often has a shorter window because testing commonly targets parent THC. Smoking and vaping leave THC in the mouth, after which concentrations usually fall as oral contamination clears and THC declines in the body.
There is no single reliable saliva window. In a controlled study of 14 frequent and 10 occasional smokers, participants smoked one 6.8% THC cigarette and provided oral-fluid samples for up to 30 hours. Criteria that required THC plus CBD or CBN produced the shortest last-detection times, roughly 1 to 13.5 hours. THC-only criteria and rules incorporating THC-COOH produced longer windows.
At a proposed THC cutoff of 2 micrograms per liter, median last-positive times were 21 hours in frequent smokers and 13.5 hours in occasional smokers, with individual results extending to the study’s 30-hour collection limit. Those findings apply to the tested device, dose, thresholds, and population. They are not a guarantee that every saliva test will turn negative within a day.
Collection systems also differ in the volume recovered, specimen dilution, drug recovery from the pad, and testing buffer. A roadside device, workplace laboratory assay, and clinical method may therefore produce different results from the same person at the same time.
An unusual chronic-use finding shows why categorical promises are risky. In a monitored detoxification study of 26 chronic users, oral-fluid THC was detected in 11 participants. Negative samples were sometimes followed by positives, and THC appeared for up to eight days after admission. This exceptional result should not be converted into a routine eight-day saliva window.
For limited use, 6 to 24 hours is a rough general estimate, and some methods extend into the 24 to 48-hour range. Frequent use, a sensitive threshold, and a different analyte may extend detection. A claimed same-day pass is an anecdote unless the dose, device, cutoff, collection, and confirmed result are documented.
How Long Does Weed Stay in Blood?
Blood is associated with recent exposure because parent THC enters circulation quickly and then falls sharply after smoking or vaping. However, blood THC does not map consistently to impairment. Route, timing, tolerance, and individual physiology affect both concentration and behavior.
After inhalation, THC commonly peaks near use and declines substantially over the next one to four hours. Edibles are slower and more variable. In a controlled study, 18 adults who had not used cannabis for at least three months consumed brownies containing 10, 25, or 50 mg THC, with six participants assigned to each dose. Whole-blood THC detection ranged from 0 to 22 hours at a sensitive 0.5 ng/mL quantitation limit.
Frequent long-term use can leave low residual concentrations after acute effects have ended. In a seven-day monitored-abstinence study of 25 frequent users, 16 had at least one whole-blood THC result at or above 0.25 ng/mL. On day seven, six participants still had detectable THC. Nine had no measurable THC at any point, which shows the size of the variation even within a selected frequent-use group.
Blood may also be tested for active 11-hydroxy-THC or inactive THC-COOH. Parent THC and 11-hydroxy-THC generally support more recent exposure than THC-COOH, but no single concentration can identify the exact consumption time or prove behavioral impairment in every person.
How long does weed stay in blood? After a single dose, the answer is commonly several hours and sometimes about a day at sensitive thresholds. Low residual THC can remain detectable for days in some frequent users. Interpretation requires the stated analyte and threshold.
How Long Does Weed Stay in Your System in Common Scenarios?
A phrase such as “one night” or “a couple of puffs” still leaves out the dose, recent use history, specimen, and cutoff. The table below gives evidence-informed planning ranges for isolated use by an infrequent user. Researchers did not directly measure every scenario as worded, and none of these ranges guarantees a test result.
| Scenario | Urine | Saliva | Blood | Hair |
|---|---|---|---|---|
| A couple of puffs after no recent use | Same day to roughly 1 to 3 days at a 50 ng/mL screen, as an evidence-informed estimate | Commonly hours to about 24 hours; some methods extend to 24 to 48 hours | Usually hours and sometimes about a day at sensitive thresholds | A 3 cm sample has a roughly 90-day retrospective window, but one light exposure may be missed |
| One isolated night | Same day to roughly 1 to 3 days at 50 ng/mL; older higher-dose data at a lower 15 ng/mL cutoff approached 4 days | Commonly hours to about 24 hours; sometimes 24 to 48 hours depending on the cutoff | Usually hours and sometimes about a day at sensitive thresholds | Roughly 90 days for a 3 cm segment, but one-time use may not be detected |
| One weed gummy after no recent use | Same day to roughly 1 to 3 days at 50 ng/mL, as a practical estimate; oral use does not create one fixed longer window | Usually hours and sometimes about a day in controlled oral research | Usually hours and sometimes about a day at sensitive research thresholds | Roughly 90 days for a 3 cm segment, with limited sensitivity to one light exposure |
| One weed-pen session after no recent use | Same day to roughly 1 to 3 days at 50 ng/mL, as a practical estimate; dose can vary substantially | Commonly hours to about 24 hours; sometimes 24 to 48 hours depending on the cutoff | Usually hours and sometimes about a day at sensitive thresholds | Roughly 90 days for a 3 cm segment, but light use may be missed |
| Comparable use by an athlete or lean person | No validated shorter range based on fitness or body fat | No validated athlete adjustment | No validated athlete adjustment | No validated athlete adjustment |
After a Couple of Puffs
After a couple of puffs with no recent use, urine may remain detectable from the same day to roughly 1 to 3 days at a 50 ng/mL screen. That is an evidence-informed planning estimate, not a range measured after exactly two puffs. Saliva is commonly measured in hours to about 24 hours and sometimes 24 to 48 hours, while blood is usually measured in hours and sometimes about a day at sensitive thresholds.
In a controlled study of smoked and vaporized cannabis in 17 infrequent users, participants received measured 10 or 25 mg THC doses rather than a loosely defined number of puffs. Using a 50 ng/mL urine screen and 15 ng/mL confirmation, at least one positive specimen occurred in 21% of smoked sessions and 47% of vaporized sessions across active doses. Collection ended after eight hours, so the study supports possible same-day detection but cannot define the full window after two specific puffs.
A 3 cm hair segment can provide an approximately 90-day retrospective window once the exposed hair grows above the scalp. However, hair testing may miss a single light exposure.
After One Night
After one isolated night of cannabis use, an evidence-informed urine planning range is the same day to roughly 1 to 3 days at a 50 ng/mL screen. Saliva is commonly detectable for hours to about 24 hours and sometimes 24 to 48 hours, while blood is usually detectable for hours and sometimes about a day at sensitive thresholds.
In the controlled Huestis, Mitchell, and Cone urine study, mean detection at a 50 ng/mL screen was less than one day after the lower-potency cigarette and less than two days after the higher-potency cigarette. At a lower 15 ng/mL analytical cutoff, mean last detection extended to 33.7 and 88.6 hours, with the higher-dose mean approaching four days.
One night can include repeated smoking, vaping, or edibles, so total THC and recent use history matter more than the calendar label. Hair can potentially record the exposure within a roughly 90-day segment, but it is less reliable for one-time or light use.
Weed Gummies
After one gummy with no recent use, an evidence-informed urine planning range is the same day to roughly 1 to 3 days at a 50 ng/mL screen. Saliva and blood are usually measured in hours and sometimes about a day at sensitive thresholds. Hair has a roughly 90-day retrospective window for a 3 cm segment, but one light exposure may be missed.
Gummy effects may begin approximately 30 to 120 minutes after ingestion. In the controlled edible study cited above, effects peaked 1.5 to 3 hours after ingestion and lasted 6 to 8 hours, while blood and oral-fluid THC were detectable for up to 22 hours at sensitive research limits.
A separate closed-unit study comparing smoked, vaporized, and oral cannabis in occasional and frequent users found that route changed the timing and pattern of urinary cannabinoids. It also found major differences between occasional and frequent users.
Oral use does not create one fixed longer detection window. Total THC and prior use frequency are more important than the word “gummy,” while package accuracy, absorption, analyte, and cutoff add uncertainty. The 6 to 8-hour effects range is not a urine-test estimate.
Weed Pen
After one weed-pen session with no recent use, an evidence-informed urine planning range is the same day to roughly 1 to 3 days at a 50 ng/mL screen. Saliva is commonly measured in hours to about 24 hours and sometimes 24 to 48 hours, while blood is usually measured in hours and sometimes about a day at sensitive thresholds.
A weed pen can feel brief while still delivering a substantial THC dose. Cartridge potency, puff duration, device temperature, and inhalation technique make “one hit” or “one session” an unreliable dose description.
The infrequent-user smoked-versus-vaporized urine study found qualitatively higher peak THC-COOH concentrations and more positive specimens after vaporized cannabis than after the same labeled smoked dose. At least one specimen reached the 15 ng/mL confirmation threshold in 74% of active vaporized sessions compared with 35% of active smoked sessions. Under the full 50/15 federal-style criteria, 47% of vaporized sessions and 21% of smoked sessions produced a positive urine specimen.
The study collected urine for only eight hours, so it does not directly establish a 1-to-3-day pen window. It does show that vaping should not automatically be assigned a shorter window than smoking. Repeated pen use can produce the longer patterns associated with frequent exposure, and hair testing may miss one light session despite a roughly 90-day lookback.
Athletes and Skinny People
Athletes and people with low body fat do not have a proven fast-clearance category. For a comparable isolated dose, use the same evidence-informed specimen ranges shown above rather than shortening them because of body type. THC is fat-soluble, so fat storage contributes to its long terminal elimination, but that mechanism does not let a lean person predict an earlier negative result.
Training status, muscle mass, body-fat percentage, resting metabolism, and sweat rate are not enough to calculate when a specimen will fall below a cutoff. Frequency, dose, prior duration of use, specimen type, analyte, urine concentration, and program rules remain more informative.
Exercise does not resolve the uncertainty. A small cycling study found a temporary increase in plasma THC after exercise, while a second small study found no major serum or urine change after moderate exercise in abstinent daily users. Neither study established faster clearance in athletes or a dependable change in urine results.
What People Say
Why do forum timelines extend far beyond laboratory charts? Personal reports combine uncontrolled doses, uncertain histories, different tests, and incomplete documentation. They can show what people experience, but they cannot calculate your detection window.
- Urine, unverified: One Reddit poster reported positive home tests after more than 80 days. The post does not verify abstinence, specimen concentration, home-strip performance, cutoff, or laboratory confirmation.
- Saliva, unverified: One poster reported passing a laboratory mouth-swab test six hours after a vape puff. The full use history, dose, device, cutoff, and confirmed analytical result were not independently documented. It is one report, not a reliable six-hour window.
- Hair, unverified: One poster reported passing after 63 days, while another reported failing after 125 days. Neither report establishes growth rate, sample site, segment length, analyte, decontamination, cutoff, or confirmation procedure.
Home urine strips add another layer of uncertainty. They usually provide a visual screening result at one cutoff, not an exact THC-COOH concentration. Laboratories may use a separate initial method, confirm a specific metabolite, assess specimen validity, and apply program-specific reporting rules.
Reported abstinence is also difficult to verify outside monitored research. Dates may be remembered incorrectly, potency may be unknown, and delta-8 THC or other cannabinoid products may be omitted. A story can be sincere and still provide a poor estimate for someone else.
Use anecdotes to understand the range of concern. Use controlled studies and the actual testing program to interpret a result.
Why Weed Is Detectable for So Long
The psychoactive effects can end within hours while THC metabolites remain detectable for days or weeks. This gap begins with THC’s lipophilicity, meaning it dissolves readily in fat rather than water.
After cannabis enters the bloodstream, THC distributes rapidly into the brain and other tissues with a strong blood supply. It then moves into less-perfused tissue and body fat. The liver converts THC into 11-hydroxy-THC, an active metabolite, and then THC-COOH, an inactive metabolite commonly measured in urine.
The body further processes THC-COOH through glucuronidation, which attaches a water-soluble group that helps the metabolite leave the body. Excretion occurs mainly through feces, with a smaller portion leaving through urine.
Stored THC and related compounds can slowly redistribute from tissue into blood. Possible enterohepatic circulation, in which compounds excreted into bile are reabsorbed from the intestine, may contribute to the long terminal elimination phase. A review of human cannabinoid pharmacokinetics describes rapid distribution followed by slower release and elimination.
The concentrations in this terminal phase are generally much lower than the peaks shortly after use. Even so, a sensitive assay may detect them. Repeated use can add new doses before earlier THC has cleared, increasing tissue stores and the amount of metabolite available for excretion.
Each specimen therefore answers a different question:
- Blood and saliva commonly measure parent THC and are more closely associated with recent exposure.
- Urine usually measures inactive THC-COOH after the body has processed THC.
- Hair may contain cannabinoids incorporated during growth or deposited from sweat, sebum, or the environment.
Summary: Prolonged detectability reflects slow redistribution, metabolism, and specimen-specific measurement. It does not mean cannabis causes impairment for the entire detection window.
What Influences These Timings
The fastest way to improve an estimate is to identify the test conditions before counting days. These factors create the largest differences:
- Frequency and duration of use. One exposure generally produces a shorter urine window than daily use over months. Repeated dosing can increase tissue stores and THC-COOH excretion.
- Dose and potency. A larger THC dose gives the body more THC to distribute and metabolize. Product labels, serving sizes, and inhaled doses are often uncertain, so potency cannot provide an exact countdown.
- Route of administration. Smoking and vaping produce rapid peaks. Edibles absorb more slowly, undergo extensive liver metabolism, and produce delayed, variable concentration curves.
- Specimen, analyte, and cutoff. Urine THC-COOH, oral-fluid THC, blood THC, and hair cannabinoids are not interchangeable. A lower analytical threshold can extend detection. Under federal workplace urine rules, screening and confirmation are separate steps.
- Collection timing and testing program. A report depends on when the specimen was collected and how the program defines a positive. Court, clinical, workplace, and home tests may follow different rules.
- Hydration and urine concentration. Fluid intake and normal biological variation alter how concentrated a urine specimen is. Evidence does not show that ordinary changes in fluid intake accelerate liver metabolism, release THC from tissue on a predictable schedule, or shorten the underlying elimination process. Creatinine and specimen-validity measurements help laboratories and researchers recognize concentration changes. Adequate hydration supports normal health, but it is not a reliable clearance strategy and cannot guarantee a result.
- Athletic status, body composition, and metabolism. THC storage in fat is biologically relevant, but being athletic, muscular, lean, or “skinny” does not establish a shorter detection window. Frequency and duration of use are more useful for interpretation than body composition, and no controlled evidence supports a separate athlete timetable. Genetics, age, liver or kidney health, medication use, and metabolic variation may contribute. No validated consumer formula combines these variables into a negative date.
- Exercise. Evidence is limited and mixed. A study of 14 regular users found a small, temporary rise in plasma THC after 35 minutes of cycling. A study of six abstinent daily users found no major serum or urine changes after 45 minutes of moderate exercise. These small studies did not establish faster elimination, a shorter urine window, or a predictable benefit for athletes.
- Hair-specific variables. Growth rate, collection site, sample mass, segment length, cosmetic treatment, melanin binding, environmental exposure, decontamination, analyte selection, and cutoff can affect interpretation.
For a high-stakes result, identify the specimen, analyte, initial cutoff, confirmation cutoff, and program rules first. Then consider frequency, dose, duration, route, and collection timing. A laboratory professional, clinician, or qualified medical review officer can interpret a specific report more responsibly than an online calculator.
The Bottom Line
The shortest practical THC detection windows usually occur in blood and saliva, where tests often target parent THC and recent exposure. Urine can remain positive for days after infrequent use and for weeks or longer after sustained use because it usually measures inactive THC-COOH.
Hair can examine approximately three months of growth when a laboratory tests a 3 cm scalp segment. However, it may miss very recent use and is less sensitive to light use.
No calendar date guarantees a negative result, and detection does not prove impairment. For a high-stakes interpretation, use the actual specimen, analyte, screening and confirmation cutoffs, collection time, and program rules. Request those details from the laboratory or program administrator before drawing conclusions.
Frequently Asked Questions
Does a positive cannabis test mean the person is impaired?
No. Urine and hair mainly support prior exposure. Blood and oral fluid are associated with more recent use, but concentrations do not map consistently to impairment. SAMHSA distinguishes a drug-test result from a fitness-for-duty determination.
Why can detectable THC still be reported as negative?
An instrument may measure an analyte below the program’s positive cutoff. Federal urine testing, for example, uses a 50 ng/mL initial marijuana-metabolite screen and 15 ng/mL confirmation. A result must satisfy the defined reporting process, so detectable and reportable positive are not synonyms.
Can body fat predict when a cannabis test will be negative?
No. THC storage in fat helps explain slow elimination, but body-fat percentage does not account for dose, frequency, route, analyte, cutoff, urine concentration, or individual metabolism. No validated formula can convert body composition into an exact negative date.
Does exercise clear THC faster?
Current evidence does not support exercise as a reliable way to shorten THC detection. Two small studies produced different blood findings, and neither demonstrated faster clearance or a predictable urine result. Exercise may support general health, but it cannot guarantee a testing outcome.
Can a negative urine test be followed by a positive without new use?
Yes, particularly near the cutoff during early abstinence after frequent use. Monitored research documented negative specimens followed by later positives. Changes in urine concentration and intermittent residual metabolite excretion can move results across a threshold. A qualified reviewer needs confirmation details and serial creatinine-normalized values to interpret the pattern.