Coding-Decoding

Every SSC CHSL reasoning paper has a coding-decoding question. The chapter looks intimidating only until you see the four standard patterns. After that, it becomes one of the most rewarding sections in the paper.

Definition: Coding is the process of converting a word or message into a secret form by following a rule. Decoding is the reverse — recovering the original word from its coded form.

Definition: The rule is the heart of every coding question. The exam never asks you to invent a rule; it shows you one example, and your job is to find the rule and apply it to a new word.

The four classical patterns

In SSC CHSL, the General Intelligence section serves coding-decoding questions that fall into four well-known patterns. Memorising the patterns once gives you a lifetime advantage.

Letter shifting moves every letter forward or backward by a fixed number in the English alphabet. If CAT becomes DBU, look closely: C→D is +1, A→B is +1, T→U is +1. The shift is constant (+1) and the rule is clear. Some questions shift backwards: CAT → BZS (each −1). Some are larger shifts: CAT → HFY (+5). The skill is identifying the gap quickly.

Reverse coding writes the word backwards. CAT → TAC. SCHOOL → LOOHCS. There is no math, no shift — just a mirror. It is the easiest of the four families, and it shows up often in SSC CHSL because it tests whether you read the example carefully or assume a shift.

Number coding replaces each letter by its position number in the English alphabet: A=1, B=2, C=3, … Z=26. CAB becomes 3, 1, 2. BAD becomes 2, 1, 4. The reverse scheme (A=26, B=25, … Z=1) is also common — and that's a popular trap. Always verify which scheme the example uses before you decode the target.

Substitution is the trick family. Whole words get redefined: "If apple is called mango, mango is called banana, banana is called orange, then what do monkeys love to eat?" The answer is banana, because the original food mango has been renamed banana in this puzzle. There is no calculation — just disciplined reading.

Why this chapter is a scoring goldmine

SSC CHSL Tier-1 has 25 reasoning questions worth 50 marks. Coding-decoding alone usually contributes 2-4 marks. These marks are entirely about pattern-spotting; they need no advanced math or vocabulary. A candidate who has practised 50-60 coding questions in their lifetime will recognise the family within 5 seconds and solve within another 20.

The hidden benefit is time saved. If coding sets cost you only 25 seconds each, you have spare minutes to spend on tougher Series and Puzzle sets. That redistribution can lift your reasoning score by 3-5 marks overall, which often makes the difference between a Tier-2 callup and a near miss.

The position-number rough line

Before you start any coding section, mentally scribble A=1, B=2, …, Z=26 across a rough line in your notebook. It takes 10 seconds. Combined with the EJOTY anchor (E=5, J=10, O=15, T=20, Y=25), counting any letter's position becomes near-instant. For example, if you need P, jump to O=15 and add 1 → P=16. If you need M, jump to J=10 and add 3 → M=13.

The same trick works backwards. To find Z=26, V=22, S=19 in the reverse scheme, just subtract from 27: A=27−1=26 reverse, B=27−2=25 reverse, and so on.

Real-world example: Indian Railways uses station codes like NDLS (New Delhi) and CSTM (Chhatrapati Shivaji Terminus). These are not strict letter shifts, but the principle — a short coded form representing a longer name — is the same substitution logic that coding-decoding uses in your exam. The mental habit of "decode the rule, then apply it" is the same one a station master uses every morning.

Question: If WHITE is coded as XIJUF, find the code for BLACK.
Solution:
Step 1: Compare W→X (+1), H→I (+1), I→J (+1), T→U (+1), E→F (+1).
Step 2: The shift is constant at +1.
Step 3: Apply to BLACK — B→C, L→M, A→B, C→D, K→L.
Conclusion: BLACK is coded as CMBDL.

Common misconception: "Reverse coding and letter shifting look similar — I'll guess." They are completely different families. Reverse coding produces a word the same length, with the letters in the opposite order; letter shifting produces a word the same length but with each letter advanced or retreated. If you can't tell them apart, count the matching letters position-by-position. If position 1 of the code equals the last letter of the original, it's a reverse code. If position 1 of the code is one or two letters away from the first letter of the original, it's a shift. A 5-second check beats a guessed answer.

A four-step decoder for any question

1. Read both the example and the target word slowly.
2. Compare lengths. Same length → either shift, reverse, or number coding. Different length → almost always substitution or number coding.
3. Test for reverse. Check if the code reads as the original spelled backwards. If yes, you're done.
4. Test for shift. Find the gap for each letter. If constant, apply that gap to the target. If not constant, suspect an alternating shift.

Most SSC questions resolve at step 3 or step 4. This routine takes well under a minute.

Q: If CAT is coded as ECV, how is DOG coded?

Step 1: Compare CAT with ECV.

• C(3) -> E(5): +2
• A(1) -> C(3): +2
• T(20) -> V(22): +2
  Rule: each letter moves +2.

Step 2: Apply +2 to DOG.

• D(4) -> F(6)
• O(15) -> Q(17)
• G(7) -> I(9)

Answer: FQI.

Tip: If a letter goes past Z, wrap around to A (Z+1 = A). Always verify the SAME shift works for every letter of the example before applying it.

Coding-Decoding gives words or numbers written in a secret "code"; you decode the rule and apply it. SSC CHSL Tier-1 reliably carries 2–4 questions, making it one of the highest-yield reasoning topics. The rule is almost always a fixed letter shift, a reversal, or a position-number operation.

The main types

1. Letter-shift coding — each letter moves a fixed number forward/back (CAT → DBU is +1).
2. Reverse coding — the word is written backwards, sometimes with a shift.
3. Number coding — letters replaced by position numbers (A=1…Z=26) or by a formula.
4. Symbol/substitution coding — whole words swapped for other words.

Code seen	Rule	Decode
CAT → DBU	+1 to each letter	shift back 1
DOG → GOD	reverse	read backwards
A=1, B=2 …	position values	map back
"sky is blue"→"pa ni ta"	word substitution	match by elimination

Exam Tricks & Tips

• 🎯 Write the alphabet with numbers 1–26 on rough sheet at the start — every letter-shift becomes instant addition/subtraction.
• 🎯 Check the gap between corresponding letters of the plain word and the code; a constant gap = simple shift.
• 🎯 If forward shift fails, test backward and reverse — most CHSL codes are +n, −n, or reversal.
• 🎯 For substitution coding, line up the common word across two statements to fix its code, then eliminate.
• 🎯 Remember EJOTY (E5, J10, O15, T20, Y25) to jump to a letter's position without counting from A.
• ❌ Common mistake: forgetting the alphabet is cyclic — after Z you wrap to A (Z +1 = A), so keep shifts within 1–26.

Expected exam pattern

"If CAT is coded as DBU, how is DOG coded?", number-code and word-substitution variants. Usually 2–4 marks and fast once the shift is found.

Quick recap

Find the fixed relationship between plain letters and code (shift, reverse, or position number), remember the alphabet is cyclic, use A=1…Z=26 and EJOTY, and apply the same rule to the target word.

Cover the answer, decode the rule, then check. 12 cards for SSC CHSL coding-decoding.

Q1. The single most useful setup for coding questions?
A1. Write the alphabet with position numbers (A=1…Z=26) so every shift is simple addition/subtraction.

Q2. CAT is coded DBU. What is the rule?
A2. +1 to each letter (C→D, A→B, T→U).

Q3. Using +1, how is DOG coded?
A3. EPH (D→E, O→P, G→H).

Q4. What is the EJOTY trick?
A4. Anchor letters at multiples of 5: E=5, J=10, O=15, T=20, Y=25 — jump to any letter fast.

Q5. Why must you treat the alphabet as cyclic?
A5. After Z it wraps to A. So Z + 1 = A, keeping all positions within 1–26.

Q6. DOG is coded GOD. What is the rule?
A6. Reversal — write the word backwards.

Q7. Reverse-alphabet code of a letter at position n?
A7. 27 − n (A↔Z, B↔Y, …), i.e. the "opposite" letter.

Q8. If forward shift doesn't match, what two rules do you test next?
A8. Backward shift (−n) and reversal (with or without a shift).

Q9. In word-substitution coding, how do you fix a word's code?
A9. Find the common word shared by two statements; its code is the common code word. Then eliminate.

Q10. Code for "5, 1, 20" if letters are position numbers?
A10. E, A, T → the word EAT (A=1,E=5,T=20).

Q11. MANGO with +2 shift becomes?
A11. OCPIQ (M→O, A→C, N→P, G→I, O→Q).

Q12. Fastest way to test a two-word code pair for a shift?
A12. Compare letter positions of first letters; if the gap is constant across all letters it is a uniform shift.