Letter codes：全部卡片

Its position in the alphabet, counting A as 1

提示A is 1, B is 2 — carry on the same way.

为什么B is 2, C is 3, all the way to Z at 26. Numbers decode back into letters the same way — and the worked pair in the question shows you the system before you use it.

How far each letter has shifted along the alphabet

提示Compare the worked pair — count the jump, then use that same jump on the new word.

为什么The move is fixed: if A becomes C, every letter goes two places forward. It can also run backwards, so check the direction from the example before you start.

Undo the shift, moving each letter back the same distance

提示Whatever the code did, do the reverse to return home.

为什么One worked pair fixes the shift. To encode, apply it; to decode, reverse it — two directions of the same rule.

The alphabet wraps round and carries on from A

提示Picture the letters written in a circle rather than a line.

为什么After Z comes A again, and before A comes Z — so a shift of two forward turns Y into A. Watch out: letter SEQUENCES are different, and never wrap.

5

提示Count up from A on your fingers — it is not far.

为什么E is a handy anchor: from it you can reach D or F in one step instead of counting from A every time.

10

提示The first letter to need double figures — a nice round number.

为什么J at ten is the anchor for the front of the alphabet: A to J is exactly ten letters, so anything nearby is one or two hops away.

20

提示J is ten — this one is twice as far along.

为什么E 5, J 10, T 20: with three anchors you rarely count more than a few letters in either direction.

26

提示The very last letter — how many letters are there altogether?

为什么Knowing the alphabet ends at 26 lets you count from the back: the 25th letter is Y and the 24th is X.

M and N

提示Thirteen letters on each side — the halfway pair.

为什么M is the 13th letter and N the 14th. Starting a count from the middle is often quicker than starting from A.

27

提示One more than the number of letters in the alphabet.

为什么C is the 3rd letter, and 27 − 3 = 24, so C is the 24th from the end. Mirror-code questions lean on this constantly.

It turns into the same new letter both times

提示One rule for all means a repeat stays a repeat.

为什么The shift never changes partway through, so a doubled letter comes out doubled. That makes repeats a free check: if your answer has lost a double, you have slipped somewhere.

Its length, and where any repeated letters fall

提示Count first, then look for a doubled position — no counting through the alphabet needed yet.

为什么A five-part code cannot belong to a four-letter word, and a code with two matching pieces must pair with a word that has two matching letters. Those two checks usually leave one option standing.

15

提示One place past the halfway pair.

为什么N is 14, so the second half of the alphabet opens here. Counting on from the middle beats counting up from A every time.

16

提示Two places past the halfway pair — and ten places past F.

为什么P anchors the middle stretch. It pairs neatly with F at six, because the two sit exactly ten letters apart.

The jumps between letters, not the letters themselves

提示Moving from one place to the next counts as one move, however many names you touch.

为什么From D to F is two moves, not three — the starting letter is not a step. This off-by-one slip is the commonest wrong answer in the whole strand.

Put your answer through the rule again and see whether the original returns

提示A correct undo, done forwards once more, lands you exactly where you began.

为什么Encoding and decoding are one rule in opposite directions, so a wrong shift shows up instantly when you reverse it. Ten seconds of checking saves a whole question.

Something has gone wrong — no single letter reaches beyond twenty-six

提示The alphabet runs out at Z, so nothing can sit above it.

为什么Every letter lives somewhere from 1 to 26, so a bigger number means a mis-count, or two numbers read as one. Check the spacing before you count again.

Z, Y and X

提示Fold the alphabet in half and see which letters meet.

为什么Each pair adds up to 27, which is why taking a position away from 27 gives its mirror. Learn the first three pairs by heart and the rest come quickly.

All 26 letters in a row with their numbers underneath

提示Spend thirty seconds at the top of the page and buy yourself accuracy for the rest of it.

为什么A drawn-out line removes the counting slips that cost marks, and adding the position numbers underneath means both kinds of code question are served by the same strip of paper.

Go two places back instead — both routes land in the same spot

提示Round a circle, a long journey one way equals a short one the other.

为什么Because the alphabet wraps, a shift of 24 forwards and a shift of 2 backwards give the same result. Whenever a shift looks large, take 26 away from it and work with what is left.

Yes — the shift is applied to each letter on its own

提示Length changes nothing; every character gets the same treatment.

为什么A code is a rule about single letters, not about whole words, so it copes with any length. Work left to right and finish every letter before you read the word back.

How many numbers there are against how many letters the answer needs

提示Four pieces cannot build a word of five.

为什么Each number stands for exactly one letter, so the two counts must match. If they do not, a two-digit number has probably been read as two single ones.

One letter can fit by luck; a second one proves the rule

提示Two agreements beat one — check before you commit.

为什么A shift that matches on the opening letter and fails on the next was never the rule at all. Confirm on a second letter, and only then work through the word you are asked about.

The nearest anchor you know, forwards or backwards

提示Never trudge up from the beginning if a landmark sits closer.

为什么With E at 5, J at 10, T at 20 and Z at 26 in your head, no letter sits more than three steps from a landmark. Counting three is quick and safe; counting twenty is neither.