Letter Series Test
Five letters follow a rule and the sixth is missing. A free online letter series test — also called an alphabet series test: 12 series, 4 options each, a few minutes, no account, everything generated and scored in your browser. Every answer is explained the moment you give it — right or wrong, you get the rule in its own letters, the wrap past Z included — and at the end the whole run is listed out: each series, what you picked, what the answer was, and why. The result itself is your count, your mean time and the rules that held you up.
Take the test
Find the rule behind the five letters, then pick the sixth. Keys 1–4 work too.
E G I K M ?
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of 12 series solved
A clean run: the alphabet stopped being a list and turned into positions you could count in, including where a series ran past Z and picked up again at A. Time is the thing left to improve.
Most of the run came apart. What usually survives is the interleaved rule, where the letter next door tells you nothing and only every other one carries the step.
About half. That is the ordinary shape of a first run — counting places in the alphabet is its own small skill, and the same three rules come back on the next one.
Fewer than half this time. One run of 12 in a browser tab says nothing about you in general — it is a warm-up, and the same three rules come back on the next one.
- Same step
- — / 5
- Growing step
- — / 4
- Two series interleaved
- — / 3
Each explanation holds for up to 4.3 s — click the card, or press Space or Enter, to move on sooner.
12 series, easy ones first, each one explained as soon as you answer. There is no time limit — the clock exists only so the result can tell you how long you took. Nothing is uploaded; it all stays in this tab.
Your answers, with the rule for each
Every series of the run you just took: what you were shown, what you tapped, what the answer was, and the rule behind it. The rules are read off the items themselves, so nothing here is a general description of the format — it is what these 12 were actually built from.
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
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Your answer Correct answer
That was a single run. In the app it becomes a short daily session, with a Focus Index to show for it.
Point your camera at the code — the App Store opens.
What a letter series test measures
Every series here has exactly one rule behind it. You are shown five letters it produced and asked for the sixth — so the work is to infer the rule from instances and then apply it once.
Psychologists call that pattern induction, and it is the most common way of getting at fluid reasoning: solving something you were never taught a method for, as opposed to recalling something you learned. What the letters add over the same task in numbers is a second step. A letter is not a quantity, so before any rule can be tested it has to become a position — G is the 7th letter — and the whole series has to be held that way while you look for the pattern.
The alphabet also ends. Positions are counted around a circle of 26 letters, so a series that reaches Z carries on from A, and more than a third of the ones in a run do. That wrap is the single most common reason a rule someone had already found still produced a wrong answer — and it is why this test is not simply the number-series test with different symbols.
What it does not measure is vocabulary or spelling. The letters never make a word, no series is a word scrambled, and knowing English better than the next person is worth nothing here. If a series beats you it is because the rule stayed hidden.
The three rules you will meet
12 series to a run, drawn fresh each time from these three and served easy ones first. Each example below is a real draw from the same generator the test uses — the sixth letter, the one the test withholds, is shown in color.
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Same step
H J L N P R
Every letter moves +2: H · J · L and on to R. The gap never changes, so once you have counted it the last letter is counting rather than insight.
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Growing step
H K P W F Q
The step itself grows — +3, +5, +7, +9, +11 — so the sixth letter is Q. It runs past Z and picks up again at A. Irregular at a glance, perfectly regular one level down: take the gaps between the gaps and the rule is in front of you.
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Two series interleaved
M Q N S O U
Two series taking turns: M · N · O steps +1, Q · S · U steps +2. Consecutive letters tell you nothing here; every other letter tells you everything.
Where letter series came from
Older than the computers that now generate them, and one of the few test formats that has been in continuous use for most of a century.
- 1938
Thurstone puts series into the model
L. L. Thurstone argued intelligence was not one thing but several primary mental abilities, and built a test for each. Letter series were among the tasks he used for the reasoning factor — five or six letters, one rule, name the next.
- 1956
A lifetime of the same task
K. Warner Schaie began the Seattle Longitudinal Study on Thurstone-style tests, letter series included, and kept measuring the same people for decades. Much of what is known about how inductive reasoning changes with age rests on this format.
- 1963
A program solves them
Simon and Kotovsky wrote a computer model that induced the rules behind Thurstone letter series. It made the task a favorite of cognitive science: to program it you must first say exactly what a person does, step by step.
- Today
Generated, not written
Series like these are produced by an algorithm from a handful of parameters — which is how a test can hand out 12 of them without anyone authoring a single item. The set on this page is drawn in your browser by a port of the generator NEURA uses.
Practicing letter and alphabet series
The format answers to more than one name — alphabet series in exam preparation books, letter sequences elsewhere — and whichever one you searched for, the practice that helps is the same.
Most of it is a single habit: stop reading letters and start reading positions. Take the gaps between neighboring terms first, D to G being +3, and a fixed step gives itself away at once. If the gaps themselves grow, you are one layer down and the differences between the differences will be constant. If neither holds, stop reading the series as one line and read every other letter instead — two progressions taking turns look like noise until they are split apart.
The second habit is remembering the wrap. More than a third of the series in a run pass Z and carry on from A, so a step counted straight down the alphabet produces a letter that is not on offer. When your answer is not among the 4 options, check the wrap before you doubt the rule.
What practice does not do is grow into something broader. You will get quicker at turning letters into positions and quicker at spotting which of the three shapes you are looking at, which is worth having if you are preparing for a test that uses this format — and worth being honest about if you were hoping for more than that.
How to read your result
Three numbers, and they are worth more together than apart.
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The count
How many of the 12 you solved. With 4 options per series, tapping at random is worth about 3 of them, so the count only starts saying something above that — and a run of guesses shows up in the time as plainly as in the number.
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The per-rule tally
Which rules actually came apart. Every same-step series solved and the interleaved ones missed is a different result from the reverse, even when the count is identical — and it tells you what to look at next time.
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The mean time
Seconds per series you solved. Wrong answers are left out on purpose: a fast wrong answer is a fast wrong answer, and folding it in would make giving up quickly look like fluency.
The verdict word under your score is a rough banding of the count and nothing more — the cuts are at 11, 8 and 5 solved. It exists so the number has a sentence attached to it, not because four bands are a finding. Two runs half an hour apart will often land in different bands, which is itself the most useful thing this page can tell you about how much a single run is worth.
What this version cannot tell you
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12 series is short
Long enough to meet all three rules and get a feel for which ones you spot, not long enough to be a measurement. A run where one lucky guess lands is a run that reads a point higher than it should.
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Counting is part of the task
Reading a letter series means turning letters into positions, and how fast you can count from R to V is not quite the same skill as spotting a rule. On a good day the two are hard to tell apart; on a tired one the counting is what slows you down.
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Your conditions are in the result
A tab in the background, a phone that buzzed, the end of a long day — all of it lands in the time and some of it in the count. Compare your own runs on the same evening rather than your run to anyone else’s.
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It is not a diagnosis or a score
Letter series appear inside professionally administered batteries, always beside other item types and read by someone qualified. One format on one page is not that, and nothing here turns your count into a score of any kind.
Questions about the letter series test
Does it show the answers and explain them?
Yes. Every one of the 12 series is explained the moment you answer it — right or wrong — with the step stated in that series' own letters, and with the wrap named whenever the series ran past Z and picked up again at A, which is the single most common reason an item gets missed. When the run ends the whole thing is listed under the card: each series as you saw it, the letter you picked, the correct letter, and the rule.
Do I need an account?
No. The letter series are generated in your browser and scored there too — nothing is uploaded, nothing is stored, and the result is gone when you close the tab. There is no email box anywhere on this page.
Is this an IQ test?
No. It is one item format — the same letter-series format that appears on reasoning and IQ tests — and it reports only what it actually measured: how many of the 12 series you solved, how long you took, and which rules gave you trouble. It does not estimate an IQ, and no single-format test in a browser tab honestly could.
What counts as a good score?
We publish no norms for our version of the letter series test, and anyone quoting a number for a browser test is guessing. Two things are worth knowing. 4 options mean about 3 of the 12 come from chance alone. And only 5 series in a run use the easiest rule, so anything above 5 means a growing-step or interleaved series came apart for you. Beyond that, the comparison worth making is with your own earlier runs.
Do the series repeat if I take it again?
Not inside one run — a run redraws rather than serve the same six letters twice. Across runs they can: the three rules have 1,120 parameter combinations between them, which is a large number to meet by accident and a small one over a hundred runs. What always stays the same is the recipe: the same three rules, the same mix of 12, and roughly the same easy-to-hard order.
Is an alphabet series test the same as a letter series test?
Yes — two names for one format, and you will also see it called a letter sequence test. Five or six letters follow a rule, one is missing, and you name it. The naming splits mostly by where you met it: exam preparation material tends to say alphabet series, psychology and the research literature tend to say letter series. Nothing about the task changes with the label, and this page runs the same thing under either one.
What happens when a series runs past Z?
A letter series that runs past Z continues from A — positions are counted around a circle of 26 letters, so the step after Y at +3 is B. More than a third of the series in a run cross that point, and it is the most common reason a rule someone had already found still produced a wrong answer.
One run is a snapshot. The trend is the point.
In NEURA, letter series are one task type inside a daily session of 20, picked for you each morning and pushed toward whatever you keep missing — and after three sessions the Focus Index opens, one score with a 14-day trend behind it. That is the part a single page cannot do.
