Coding-Decoding & Series

Syllabus Unit: General Intelligence & Reasoning

Why it matters

Chapter overview

Coding-decoding tests how fast you can reverse-engineer a substitution rule — letter shift, symbol swap, or reversal — from one worked example and reapply it exactly; series questions apply the same rule-finding skill to a single self-contained sequence instead of a paired code. This topic builds a systematic checklist for both, so no pattern search starts from a blank slate.

Tier-I Examination Syllabus General Intelligence & Reasoning 25% weightage

Live Animation1 / 4

1Read Given ExampleThe word and its code

2Find the Shift/RuleCompare each letter pair

3Verify on All LettersConfirm the rule fits fully

4Apply to New WordGenerate the required code

1Letter & Number Coding

Letter coding replaces each letter with another via a fixed positional rule — decode the rule from the given example, then apply it verbatim.

Learning Objectives

Decode a letter-shift coding rule (each letter moved +n/−n alphabet positions) from a given worked example.
Apply the opposite-letter (mirror) coding rule where position n pairs with position 27−n (A↔Z, B↔Y, ...).
Decode reversal-based coding, where a word's letters are written in reverse order.
Distinguish letter coding from number coding, where digits are substituted or shifted by an analogous fixed rule.

Key Terms

Letter Shift CodingEach letter of the word is moved a fixed number of alphabet positions forward or backward to produce the code.

Opposite Letter PairingA↔Z, B↔Y, C↔X, ... (position n pairs with position 27−n) — the mirror-coding rule.

Reversal CodingThe coded word is simply the original word's letters written in reverse sequence.

Position ValueA letter's numeric place in the alphabet, A=1 through Z=26 — the basis for most letter-coding arithmetic.

Every letter-coding question gives one worked example — the entire task is naming the exact rule (shift amount, mirror pairing, or reversal) and reapplying it identically.

CAT → DBU: C(+1)=D, A(+1)=B, T(+1)=U ⟹ the rule is 'shift each letter forward by 1'

Applying the same +1 shift to DOG: D→E, O→P, G→H, giving EPH.

Other common rule types

Opposite-letter (mirror) coding: A↔Z, B↔Y, ..., M↔N — position n pairs with 27−n.
Reversal coding: the letters of the word are simply written back to front.
Number coding: digits shifted by a fixed amount, analogous to the letter-shift rule.

Common MistakeTrap: always verify the rule against every letter of the given example, not just the first — a rule that fits only the first letter is usually the wrong rule.