Coding-Decoding

Snapshot

• Coding-Decoding disguises a word or message by a rule — a letter shift, position numbers, or a substitution — and asks you to encode or decode another. Crack the rule on the given pair and the rest follows.
• Everything rests on alphabet position (A = 1 … Z = 26) and the mirror rule (A↔Z, B↔Y; positions add to 27). Know these cold.
• This guide covers letter-shift, number, substitution and mixed coding, plus the "conditional" message type — with worked examples.
• Exam reality: +5 / −1. Find the rule on the example, then apply it mechanically.

Part 1 — The alphabet toolkit

Part 2 — Letter-shift coding

The commonest type: every letter moves a fixed number of steps forward or back.

To decode, reverse the shift. The shift can also be positional (1st letter +1, 2nd +2 …) or use the opposite letter (replace each by its mirror, A↔Z). Always test the simplest shift first.

Part 3 — Number, substitution & mixed coding

• Number coding: letters → their positions (CAB → 3-1-2), or the sum of positions (CAB → 6), or each digit transformed.
• Substitution coding: words stand for other words ("blue means sky, sky means red…"); just chase the substitution chain.
• Mixed / symbol coding: letters map to symbols or shuffled groups — line up the given code with the word letter by letter to deduce each mapping.
• Conditional message coding: a set of rules ("if a vowel is followed by a consonant, code both as #…") applied to a string — work strictly left to right.

Part 4 — Speed techniques

1. Write positions under the letters before doing anything.
2. Test +1 / −1 and the mirror (27 − position) first — they cover most letter codes.
3. For substitution, follow the chain of replacements, ignoring real meaning.
4. For symbol coding, align the two strings and read off one mapping per letter.
5. Decode by reversing the encoding rule.

Part 5 — Worked examples

1. If CAT = DBU, then DOG = ? Each letter +1 → EPH.

2. If MAN is coded 13-1-14, then SUN = ? Positions → 19-21-14.

3. Mirror code: ROSE → ? Each letter to its mirror (R↔I, O↔L, S↔H, E↔V) → ILHV.

4. If in a code "PINK = QHOL"? P+1=Q, I−1=H, N+1=O, K+1=L → mixed ±1 alternating; apply the same to a new word.

5. Substitution: "sky = blue, blue = sea". The colour of clear water is called? Water is blue, and blue = sea → sea.

6. If TEACHER = 7 (number of letters), the code is the letter count → for STUDENT it's 7 too.

7. Sum coding: CAB = 3+1+2 = 6.

8. Reverse-then-shift: if WORD is coded by reversing (DROW) and +1 (ESPX), decode ESPX → reverse the steps → WORD.

Part 6 — Common traps

• Assuming forward shift — some codes shift backward or alternate.
• Mixing position-sum with position-list — read which the example uses.
• Real meaning in substitution — ignore it; follow the rule.
• Off-by-one positions — A = 1.
• Wrap-around — after Z, shifting forward returns to A (mod 26).

Part 7 — How to use this page

Lock the positions and the mirror rule, re-solve the eight examples writing positions under each letter, then attempt the practice set and the timed test.

One-line revision: write letter positions first, test +1/−1 and the 27-mirror before anything fancy, follow substitution chains literally, and decode by reversing the rule.