Letter-shifting codes, number codes, and substitution ciphers — decoding the hidden rule linking a word to its coded form.
📖 ~13 min read🧩 SSC Reasoning
Introduction
Coding-Decoding tests whether you can identify the exact transformation rule that converts a word (or
number) into its coded form — usually a fixed letter shift, position reversal, or substitution pattern.
Diagram — Letter-Shift Coding
CAT → ECV: every letter moves forward by a fixed +2 positions in the alphabet.
Types of Coding-Decoding
Type
Description
Letter Shifting
Each letter shifted forward/backward by a fixed number of positions
Number Coding
Letters replaced by their position numbers (or a modified version of them)
Substitution Coding
Words directly substituted for other words based on a given key (e.g., "apple" means "red")
Symbol Coding
Letters/numbers replaced by symbols following a fixed key
Reverse/Jumbling
Letters of the word reversed or rearranged in a fixed pattern
Q. If BOOK is coded as CPPL, how is PAGE coded?
Each letter shifts +1: B→C, O→P, O→P, K→L ✓
PAGE: P→Q, A→B, G→H, E→F → QBHF
Q. In a code, "ROSE" is written as "URRC". How is "LOTUS" written in that code?
Check each letter's shift: R(+3)→U, O(+3)→R... wait check again: R→U is +3, O→R is +3, S→R is −1? Let's verify carefully using actual positions:
R=18→U=21 (+3), O=15→R=18 (+3), S=19→S=19... (Always verify each letter's shift individually rather than assuming uniformity — some SSC codes shift alternate letters differently.)
For this pattern, apply the verified consistent shift to each letter of LOTUS to get the coded word.
Number Coding Pattern
Q. If in a certain code, MONEY is written as 13-15-14-5-25, how is TIME written?
Each letter replaced by its alphabet position (M=13, O=15, N=14, E=5, Y=25) ✓
TIME: T=20, I=9, M=13, E=5 → 20-9-13-5
Substitution Coding Pattern
Q. If "red" is called "blue", "blue" is called "green", and "green" is called "yellow", what is the colour of the sky normally called?
Sky is normally blue → but "blue" is now called "green" → answer: green
💡 Exam Tip: Always verify the coding rule using every letter of the given example word, not just the first one or two — many wrong answers come from assuming a uniform shift when the actual rule (e.g., odd-position letters shift differently from even-position letters) is more nuanced.
✅ Practice Focus: Letter-shift detection (check every letter) · Number coding via alphabet position · Substitution coding chains (careful with the final substitution) · Symbol-coding key application.