Letter Series Puzzles
A run of letters following a hidden rule, and a gap at the end. The trick that makes these easy is the same one every time: turn the letters into their alphabet positions first, and the pattern stops being a pattern and becomes arithmetic.
Also known as alphabet series, letter sequences and missing letter puzzles.
Related Practice
Letters Are Only Their Alphabet Positions
A letter series shows a run of letters built by a single rule, with the last one missing. Your job is to work out what turns each term into the next and apply it once more. The rule might be a fixed step through the alphabet, a gap that widens, or two letters each running a series of their own.
They look harder than number series and they are not. They are the same puzzle with a layer of disguise, and the disguise comes off in one step.
Convert To Numbers First
This is the whole method. A is 1, B is 2, through to Z at 26. Write those positions under the letters and a series that looked arbitrary becomes obviously arithmetic.
D, H, L, P tells you very little. Written as 4, 8, 12, 16 it is finished in a second. Nobody reliably sees the pattern in the letters, and almost everybody sees it in the numbers, which is why this step is not optional.
| Letter | Position | Letter | Position |
|---|---|---|---|
| E | 5 | T | 20 |
| J | 10 | Y | 25 |
| O | 15 | Z | 26 |
From those six, every other letter is at most two steps away. That is a much smaller thing to memorise than the whole alphabet with numbers attached, and it does the same job.
The Five Rule Families
| Family | Example | What gives it away |
|---|---|---|
| Fixed step | D, H, L, P | The differences between positions are all equal. |
| Widening gap | B, C, E, H, L | The differences rise by one each time. |
| Letter pairs | AK, CN, EQ, GT | Each term is two letters; read them as two separate runs. |
| Both ends at once | AZ, CX, EV, GT | One letter climbs from A while the other falls from Z. |
| Alternating step | B, D, I, K, P | The jump switches between two sizes every other term. |
Why Nothing Here Wraps Past Z
Some published letter series wrap around, so that Z plus one becomes A. None here do, and that is a deliberate decision rather than an oversight.
The reason is ambiguity. If wrapping is allowed, "add four" and "add four with wrapping" agree on every term of a short run and then diverge without warning — which means a puzzle can have two defensible answers and no way for a solver to tell which was meant. Every series on this page stays inside A to Z, so the rule you find is the rule that was used.
Letter Series Puzzles Frequently Asked Questions
How do you solve letter series puzzles?
Convert the letters to their alphabet positions first — A is 1 through to Z at 26 — and then treat it as an ordinary number series. Take the differences between consecutive positions: if they are equal the rule is a fixed step, and if the differences themselves rise steadily the gap is widening. Work out the next position, then convert that number back into a letter.
What are the alphabet positions?
A is 1, B is 2 and so on to Z at 26. It is worth knowing a few anchor points by heart rather than counting from A every time: E is 5, J is 10, O is 15, T is 20 and Y is 25. From those, any letter is at most two steps away.
What kinds of rules appear in letter series?
Five cover nearly everything here: a fixed step forwards or backwards, a gap that widens by one each time, pairs of letters where each letter runs its own series, one letter walking forward from A while another walks backward from Z, and an alternating step that switches between two sizes.
Do letter series wrap around from Z back to A?
Some published series do, but none here. Wrapping makes two different readings fit the same run — "add four" and "add four with wrap" agree right up until they suddenly do not — which would leave a puzzle with two defensible answers. Every series on this page stays inside A to Z, so the rule is never ambiguous.
How do I get faster at these?
Learn the anchor letters so the conversion costs you nothing, and always write the differences down rather than holding them in your head. Those two habits between them account for most of the speed difference between people who find letter series easy and people who find them impossible.
Are letter series the same as number series?
Underneath, yes. A letter is its position in the alphabet, so a run that advances four letters is a run that adds four. Converting the letters to numbers is the whole technique, and it is why these feel easy once you stop staring at the letters themselves.
What is a letter series?
A run of letters that follows a rule based on their positions in the alphabet — A is 1, B is 2, and so on. Once you convert, "C, F, I, L" is just 3, 6, 9, 12, and the puzzle becomes ordinary arithmetic. Some series step two or three letters at a time, some alternate two patterns, and some move backwards through the alphabet.