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Concept 2 of 8

Coding & Decoding: Shift the Letters

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Today's big idea

Find the shift between one letter pair, then apply it to the whole word.

APPLE → CRRNG means shift +2. To decode, subtract 2.

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How it works

👀 What's going on?

A secret code swaps every letter for a different one. If you know the swap, the message opens in seconds. Most codes hide one simple move: slide each letter along a fixed number of places.

💡 Here's the trick

Compare original to code → find the shiftCAT+1+1+1DBUCAT → DBU · every letter shifted +1Apply +1 to a new word to decode/encode

Look at the code above. Every letter of CAT has moved one place forward to give DBU. Find that move once and it unlocks every other letter in the word.

🪄 How to do it

  1. 1.Match a letter to its code letter in the code above and count the gap.
  2. 2.Check that the same gap works on a second letter and a third one.
  3. 3.Move each letter of the new word by that gap, looping past Z back to A.

✍️ Try it

In one code, BUS becomes EXV. What is the gap? Now use the same gap to write the code for MOON.

B to E is 3 places, and U to X and S to V are 3 places too, so the gap is +3. Moving MOON on by 3 gives PRRQ.

⚠️ Watch out

You might be thinking that a gap found from one letter is enough. Test it on the rest of the word before you trust it. Some codes grow the gap as they go, so the first letter moves 1 and the second moves 2. Checking two more letters costs seconds and saves the whole answer.

🌟 Why this matters

Shifted letters turn up in party games, spy stories and the puzzle page of a magazine. Once you can spot a gap, any message written this way falls open in front of you.
Exam tip: Write the alphabet along the top of your rough sheet, then count every gap on it.

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Watch me solve one

Quanta solves it

“Watch how I think through one of these — then you try.”

THE PROBLEM

“If BRAIN is coded as DTCKP (shift +2), what is the code for CHECK?”

  1. 1

    Verify the shift

    B→D, R→T, A→C, I→K, N→P → every letter shifts +2. 🔍

  2. 2

    Apply to each letter of CHECK

    C→E · H→J · E→G · C→E · K→M

  3. 3

    Combine

    EJGEM

  4. 4

    Answer ✓

    EJGEM

Now you try one — same steps, different story 👇

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See it in action

⚡ Time to play!

Compare the sample to its code to spot the shift, then tap each letter of the new word to step it +1.

CIPHER · ROUND 1 OF 3

SUN → TVOshift +1

Tap each letter to code BOOK

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Quick check

✅ VERIFICATION

In a code, LAMP is written as NCOR. Using the same rule, what is the code for DESK?

⚡ Finish the activity above first to answer.

Keep going!

Up next: Operator Substitution

Replace symbols → apply bodmas to the new expression.

Concept 3 of 8 →