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Letter series

A, C, F, J, ? — find the rule in a run of letters, or letters with numbers, and give the next or the missing term. The rules keep coming.

  • Pattern
  • Timed
  • Endless levels

How it plays

A few terms follow a rule over the alphabet: a steady step, gaps that grow, two steps taking turns, two series interleaved, a letter with its place number (A1, C3, E5), a letter with a number on its own step.

New kinds of rule unlock as you climb; then the steps and numbers grow, fewer terms are shown, and the ? moves into the middle. No series runs past Z or back past A.

After every answer the rule is stated, with each letter’s place in the alphabet (A = 1 … Z = 26), so you can check the count yourself.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This practises counting through the alphabet and spotting a rule in a series. It is practice for letter-series questions, not a measure of intelligence.

Learn

How to get better at letter series: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Turn each letter into its place: A 1 … Z 26. Work from landmarks (E 5, J 10, O 15, T 20, Y 25) rather than counting from A.
  2. Write the gaps between neighbours. The same gap each time is a steady step; gaps growing by one are growing gaps; two gaps taking turns are alternating steps.
  3. If the gaps make no sense, read every other term. Two series may be interleaved, one going forward and one going back.
  4. With a number or a second letter in each term, treat each part as its own series — the letter, then the number — and rebuild the term at the end.
  5. With the ? in the middle, check your answer against the term after it as well as the term before.

Techniques and tricks

  • EJOTY landmarks

    E is 5, J 10, O 15, T 20, Y 25. Every letter from C to Z is at most two from one.

    e.g. R is T − 2 = 18; L is J + 2 = 12; C is E − 2 = 3.

  • Count jumps, not letters

    Moving three on from F means three arrivals — G, H, I — and F itself is not one of them. In numbers: add the step to the place.

    e.g. F + 3: 6 + 3 = 9, which is I.

  • Gaps of gaps

    If the gaps are not equal, write the gaps between the gaps. Growing by one each time means the next gap is one bigger again.

    e.g. A, C, F, J, ? — gaps 2, 3, 4, so the next is 5: J (10) + 5 = O (15).

  • Split odd and even places

    Read the 1st, 3rd, 5th terms as one series and the 2nd, 4th, 6th as another. Here one series starts in the first half of the alphabet and goes forward; the other starts in the second half and goes back.

    e.g. B, Y, D, W, F, ? — B, D, F go 2 on; Y, W go 2 back. So the ? is U.

  • Two steps taking turns

    When the gaps go small, big, small, big, the next gap is whichever comes next in the turn.

    e.g. C, D, F, G, I, ? — gaps 1, 2, 1, 2, so the next gap is 1: I (9) + 1 = J (10).

  • The number checks the letter

    In terms like A1, C3, E5 the number is the letter’s place. Work out the letter, and the number writes itself — or work the number and read off the letter.

    e.g. G7, I9, ? — two on is K, the 11th letter: K11.

  • A number with its own step

    When the number is not the place, follow it on its own: a steady step up or down, or doubling (later, tripling).

    e.g. B3, D6, F12, H24, ? — letters 2 on, numbers doubling: J48.

As the levels climb

  • You start with steady steps forward and back, five terms shown and the ? at the end. Growing gaps and alternating steps come next.
  • Then the steps get wider, only four terms are shown, and — with “Where the ? goes” on “At the end, then anywhere” — the ? can sit in the middle.
  • Interleaved series and letters with their place number arrive next; then a letter and number each with its own step, and two-letter terms; last, doubling numbers and growing gaps with the place number. After that only the size grows.
  • The clock never tightens here: each harder kind of rule and each size step adds a little time, and typing adds a quarter more. No series runs past Z or back past A, so you never wrap round.

Traps to avoid

  • The repeated gap. In A, C, F, J the last gap was 4, so N (J + 4) is offered. In a growing series the gap grows again: the answer is O.
  • One letter out. Options a place too far or too short (N or P for O) come from counting the starting letter as a jump. Count arrivals only.
  • The number one off. Beside J48 you may see J47, J49 or K49. Work the number separately once the letter is fixed.
  • Two-letter terms read as one. In terms like AZ, BY, CX, each letter has its own step and direction; following only the first letter leaves the second to a guess.

How to practise

  • Set “Where the ? goes” to “Anywhere” to practise middle gaps from the start, or “Always at the end” to learn new rules without them.
  • Switch “Answer by” to “Typing the term”. You must produce the letters and the number, so the count has to be right.
  • Lower “Options shown” to 2 while a new rule is strange; raise it towards 8 when you want more near misses to rule out.
  • Drill the alphabet’s places away from the game: name a random letter’s number from its nearest EJOTY landmark until it comes without counting.

Levels

Each question has a clock that comes from the work it asks for. There is no last level: at least one dial turns without end. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

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