SSC CGL Coding-Decoding Notes: Tricks, Types & Practice Questions

Coding-decoding is the friendliest chapter in the reasoning section: a word is 'coded' by a hidden rule, and your job is to find the rule and apply it. There are no formulas to memorise — just the alphabet table and careful checking.

SSC lists Coding & decoding explicitly in its reasoning syllabus, and Tier-1 papers regularly include a few questions on it. Because the questions become mechanical once the rule is found, this is one of the highest-accuracy topics in the paper.

These notes cover the five coding types SSC actually asks — letter shifting, reversal, number coding, substitution and matrix coding — with the alphabet table, solved examples and practice questions.

Key Concepts & Formulas

The alphabet table (non-negotiable)

Every letter-coding question is solved with A=1 to Z=26 and the opposite pairs. Memorise the table cold: you will convert letters to numbers in almost every question, and mirror codes are unsolvable without the opposites.

Formulas:

• A=1 through M=13, N=14 through Z=26
• Reverse positions: Z=1, Y=2, X=3, ... A=26
• Opposite pairs: A-Z, B-Y, C-X, D-W, E-V, F-U, G-T, H-S, I-R, J-Q, K-P, L-O, M-N

Type 1 — Letter shifting

Each letter moves forward or backward by a fixed number of positions: CAT → DBU means every letter shifts +1. Check all letters of the given word before concluding — sometimes the shift changes per letter (+1, +2, +3) or alternates (+2, −2).

Formulas:

• Fixed shift: every letter +n or −n
• Running shift: +1, +2, +3 across the letters of the word
• Alternating shift: +2, −2, +2, −2, ...
• Always verify the rule on every letter of the example word

Type 2 — Reversal and mirror

The code is the word written backwards (DELHI → IHLED), sometimes combined with a shift or with opposite letters (A–Z). If the coded word contains the same letters in reverse order, reversal is your answer.

Formulas:

• Pure reversal: DELHI → IHLED
• Reversal + shift: reverse the word, then shift each letter
• Mirror code: replace each letter with its opposite (A→Z, B→Y, ...)

Type 3 — Number coding

Letters become numbers, usually A=1 to Z=26. The code is the sum (BAT = 2+1+20 = 23), the product, or the concatenated positions. Also watch for reverse positions (Z=1 to A=26) when the normal table gives no clean pattern.

Formulas:

• Sum of positions: BAT = 2+1+20 = 23
• Reverse positions: Z=1, Y=2, ... (AT = 26+20 = 46)
• Product of positions: rare, but check it when sums do not fit

Type 4 and 5 — Substitution and matrix coding

In substitution, whole words are replaced in sentence-style statements — decode by matching the word common to two sentences. In matrix coding, letters are located by row-and-column numbers in a grid (for example 41 = row 4, column 1). For matrices, locate each letter carefully: one row/column slip ruins the whole word.

Formulas:

• Substitution: match the word common to two sentences
• Matrix: read row first, then column (41 = row 4, column 1)
• Double-check every letter's coordinates before choosing an option

Solved Examples

Practice Questions

Common Traps (galtiyan jo marks katwati hain)

• Concluding the rule from a single letter — always verify the rule on every letter of the given word before applying it.
• Alphabet wrap-around errors: Y+3 = B and B−3 = Y. Count carefully at the two ends of the alphabet.
• Assuming A=1 always — some codes use Z=1 (reverse positions). If the normal table gives no clean pattern, try the reverse table.
• In reversal codes, forgetting that the reversed word may also carry a shift — check whether plain reversal explains all the letters.
• Matrix coding: reading column-first when the question says row-first. The order stated in the question is the rule.
• Substitution traps: matching the wrong common word between two sentences. List all the words of each sentence before matching.

FAQs