Eleven Plus — Codes and Logic

Letter codes

Crack a simple letter-shift code by comparing a word and its coded version, work out the exact rule, and apply it confidently to a brand new word.

Back to Eleven Plus

💡Understand

A LETTER-SHIFT code moves every letter forward or backward through the alphabet by a fixed number of places. To crack it, compare ONE letter in the word to the SAME position letter in the code, count the shift, then check that shift works for a second letter to confirm the rule.

“If CAT is coded as DBU, compare C→D (forward 1), A→B (forward 1), T→U (forward 1). The rule is: shift every letter forward by 1.”

Checking the shift on MORE THAN ONE letter pair (not just the first) is what confirms you've found the true rule rather than a coincidence — here, all three letters shift by exactly +1, so the rule is confirmed as 'shift forward by one'.

🔍Spot it

What is the shift rule for this code?

Question 1

DOG is coded as EPH. What is the rule?

💡 Each letter moves exactly one place forward in the alphabet: D→E, O→P, G→H.

Question 2

BIRD is coded as AHQC. What is the rule?

💡 Each letter moves exactly one place backward: B→A, I→H, R→Q, D→C.

Question 3

SUN is coded as UWP. What is the rule?

💡 Each letter moves exactly two places forward: S→U, U→W, N→P.

🧠Think about it

Why is it risky to work out a code's rule from just ONE letter pair, rather than checking at least two?

✍️Try it

Crack the code and apply it to a new word.

If PEN is coded as QFO, what is the code for CAT, using the same rule?

Question 1

What is CAT coded as, using the same rule as PEN → QFO?

💡 First work out the shift rule from P→Q, E→F, N→O, then apply that exact shift to each letter of CAT.

📝Practise

Crack each code's rule, then apply it.

Question 1

If HAT is coded as IBU, what is the rule?

Question 2

Using 'shift forward by 1', what is DOG coded as?

💡 Shift each letter of DOG forward by one place.

Question 3

If FOX is coded as DMV, what is the rule?

Question 4

In one word, what should you do to a rule found from one letter pair, before trusting it?

💡 It's the key strategy emphasised in this lesson.

Question 5

Using 'shift backward by 2', what is CAT coded as?

💡 Shift each letter of CAT backward by two places: C→A, A→Y, T→R.

Question 6

Why does checking a second letter pair make your confidence in a rule much stronger?

🎯Apply

Read the passage, then answer the questions below.

Exam question: If MOON is coded as OQQP, what is the rule, and what would STAR be coded as using the same rule?

Question 1

What is the shift rule, based on M→O, O→Q, O→Q, N→P?

💡 Count how many places forward each letter moves: M to O is 2 places.

Question 2

Using 'shift forward by 2', what is STAR coded as?

💡 Shift each letter of STAR forward by two places: S→U, T→V, A→C, R→T.

Question 3

Why does checking BOTH 'O' letters in MOON (both becoming Q) strengthen confidence in the rule?

🏆Challenge

Invent your own letter-shift code (choose a shift amount), encode a four-letter word, and write out the step-by-step rule someone would need to follow to decode it.

🔄Review

A quick mastery check — 3 questions.

Question 1

If BIG is coded as CJH, what is the rule?

Question 2

Using 'shift forward by 1', what is RUN coded as?

💡 Shift each letter of RUN forward by one place.

Question 3

The safest way to confirm a code's rule is to...