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Complex span

Judge a sum true or false, then see a letter; again and again — then recall the letters in order. One more letter each level.

  • Memory
  • Untimed
  • Endless levels

How it plays

Each step shows a sum to judge (true or false) and a letter to hold. After the last one, type the letters in the order they came.

The sums keep you busy while you hold the letters — that is the point. Levels add a letter and harder sums.

Keys

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

What it trains

This practises holding information while doing something else. Gains are at this task; no wider claim is made.

Learn

How to get better at complex span: 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. Judge the sum first, by its last digit alone (the trick below). It takes a second and leaves your inner voice free for the letters.
  2. Then read the letter and say the whole list so far, from the first letter, under your breath: “F”, then “F K”, then “F K R”.
  3. Only then press True or False. There is no clock in this game, so finish the rehearsal before the next sum appears.
  4. At the end, type the letters straight away, saying each one as you type it. Capitals and spaces do not matter.

Techniques and tricks

  • Last-digit test

    Add only the last digits. A false sum here is always 1 or 2 out, so its last digit is always wrong; if the last digit matches, the sum is true.

    e.g. 17 + 8 = 24? 7 + 8 = 15, ends in 5, not 4: False. 14 + 9 = 23? 4 + 9 = 13, ends in 3: True.

  • Cumulative rehearsal

    Each time a letter arrives, say the list again from the start, not just the new letter. Every step then refreshes the oldest letter, which is the one that fades.

  • Chunk in threes

    Group the letters in threes, said with a beat: “F K R — T N P — L”. Rehearse the chunk you are building in full, and the finished ones as a quick run.

  • Add your own vowels

    The letters have no vowels, so make each chunk sayable: F K R can be “fakir”, T N P “tune-up”. Keep the letters in order inside the word; a word is easier to hold than three sounds.

  • Know the alphabet of twelve

    Every letter is one of F H J K L N P Q R S T Y, and in a set of twelve or fewer none appears twice. If you blank on one, run through the twelve and skip any you have already typed.

As the levels climb

  • At first the levels alternate: one more letter, then bigger sums. The numbers in each sum go up to 9, then 13, then 17, then 21. After that, only the letters grow, one a level, without end.
  • Bigger numbers do not slow the last-digit test: 17 + 19 is still just 7 + 9 = 16, so the sum ends in 6. Keep the full addition out of your head.
  • Past six or seven letters, saying the whole list at every step gets long. Rehearse the chunk you are building in full, and the earlier chunks once each as a block.
  • Once a set has more than twelve letters, it runs out of different ones and letters start to repeat. The no-repeat check stops working there; rely on your chunks.

Traps to avoid

  • Rushing the sums to save the letters: a wrong sum makes the whole question wrong, and each sum counts as much towards your score as the whole recall.
  • “About right” sums: the false ones are never far out, only 1 or 2, so 8 + 7 = 17 looks close enough. Check the last digit every time: 8 + 7 ends in 5.
  • Letters typed out of order: F R K for F K R. The recall is marked by position, so a swap costs two places. Rehearse in order and type in the order you say them.
  • Losing the first letter: it has been held longest, through the most sums. Cumulative rehearsal always starts from it, which is why it works.

How to practise

  • Drill the last-digit test on its own until it is instant: any two numbers you see, add the last digits. The less a sum costs, the more of you is left for the letters.
  • Leave “Feedback” on “Instant” at first: after each set, “The letters were …” shows the list, and you can see which place you lost — the first, the middle or the end.
  • Say the rehearsal out loud, quietly, while you learn it, then move to silent speech once the rhythm of “sum, letter, list” is automatic.

Levels

It is untimed unless you turn the clock on. 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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