In this guide
A coding–decoding question shows a word and its secret code, then asks you to code another word the same way. Sometimes it runs the other way and asks you to decode. Either way, the method is the same: compare the word and its code letter by letter until you can say the rule in one line.
Coding appears regularly in the SSC GD reasoning section and it is almost pure alphabet work. If you know letter positions without counting, most coding questions take under 40 seconds. That is why the reasoning plan puts it in week two, right after analogy and series.
Step zero: know the alphabet
Write A = 1 to Z = 26 on your rough sheet at the start of the exam. Use EJOTY to reach any letter fast: E = 5, J = 10, O = 15, T = 20, Y = 25.
You also need the opposite pairs. Each pair adds up to 27.
| Pair | Positions | Pair | Positions |
|---|---|---|---|
| A – Z | 1 + 26 | H – S | 8 + 19 |
| B – Y | 2 + 25 | I – R | 9 + 18 |
| C – X | 3 + 24 | J – Q | 10 + 17 |
| D – W | 4 + 23 | K – P | 11 + 16 |
| E – V | 5 + 22 | L – O | 12 + 15 |
| F – U | 6 + 21 | M – N | 13 + 14 |
| G – T | 7 + 20 |
The six common types
| Type | What happens | Quick example |
|---|---|---|
| Same shift | Every letter moves the same number of steps | BOOK → CPPL (+1) |
| Growing shift | First letter +1, second +2, third +3 … | BAT → CCW |
| Reverse | The word is written backwards | TREE → EERT |
| Reverse and shift | Written backwards, then each letter moves | RAIN → OJBS |
| Opposite letters | Each letter is replaced by its pair | ABC → ZYX |
| Number codes | Letters become positions, or their sum | CAB → 312; CAT → 24 |
A seventh type, word substitution, has no alphabet work at all. "If sky is called water, and water is called road, what do we drink?" We drink water, and water is called road. Read slowly and answer with the code word, not the real word.
The method
- Write each letter's position under the word, and under the code.
- Find the change for each letter: +1, −2, +1 +2 +3, and so on.
- If the changes make no sense, check whether the code is the word reversed, or reversed and then shifted.
- If that fails, check opposite letters (positions adding to 27).
- Apply the rule to the new word. Check one letter of your answer against the options before you mark.
Solved examples
Example 1 (same shift). If BOOK is written as CPPL, how is PEN written?
- B → C, O → P, O → P, K → L. Each letter moves +1.
- P → Q, E → F, N → O.
- Answer: QFO.
Example 2 (shift back, with wrap). If DOG is written as BME, how is CAT written?
- D (4) → B (2), O (15) → M (13), G (7) → E (5). Each letter moves −2.
- C → A. A − 2 wraps round to Y. T (20) → R (18).
- Answer: AYR.
Example 3 (growing shift). If COLD is written as DQOH, how is WARM written?
- C → D is +1. O → Q is +2. L → O is +3. D → H is +4.
- W (23) + 1 = X. A (1) + 2 = C. R (18) + 3 = U (21). M (13) + 4 = Q (17).
- Answer: XCUQ.
Example 4 (reverse and shift). If RAIN is written as OJBS, how is CLOUD written?
- R → O and A → J show no simple shift. Try reversing: RAIN backwards is NIAR.
- NIAR → OJBS. N → O, I → J, A → B, R → S. Each letter +1.
- CLOUD backwards is DUOLC. Add 1 to each: E, V, P, M, D.
- Answer: EVPMD.
Example 5 (opposite letters). If ABC is written as ZYX, how is GOLD written?
- A–Z, B–Y and C–X are opposite pairs.
- G (7) → T (20). O (15) → L (12). L (12) → O (15). D (4) → W (23).
- Answer: TLOW.
Example 6 (number code by sum). If CAT is coded as 24, what is the code for DOG?
- C (3) + A (1) + T (20) = 24. The code is the sum of positions.
- D (4) + O (15) + G (7) = 26.
- Answer: 26.
Common mistakes
| Mistake | Correct way |
|---|---|
| Finding the rule from one or two letters | Check every letter of the word before applying it |
| Forgetting to wrap round after Z or before A | Z + 1 = A; A − 1 = Z |
| Missing a reversed code | If the shifts look random, reverse the word and compare again |
| Coding when the question asks you to decode | Read which way the question runs; decoding means moving the other way |
| Answering word substitution with the real word | Give the code name, not the thing itself |
Practice set
- If SUN is written as TVO, how is MAP written?
- If FISH is written as HKUJ, how is BIRD written?
- If STAR is written as RATS, how is POOL written?
- If ABC is written as ZYX, how is HIT written?
- If BAT is written as CCW, how is SIT written?
- If CAT is coded as 24, what is the code for BOY?
- In a code, each letter is replaced by the next letter. Which word is written as DBS?
- If "pen" is called "pencil", "pencil" is called "book" and "book" is called "bag", what do we write with in ink?
Answers:
- NBQ. Each letter +1.
- DKTF. Each letter +2: B → D, I → K, R → T, D → F.
- LOOP. The word is reversed.
- SRG. Opposite letters: H (8) → S (19), I (9) → R (18), T (20) → G (7).
- TKW. Growing shift +1, +2, +3: S → T, I → K, T → W.
- 42. B (2) + O (15) + Y (25) = 42.
- CAR. Decode by moving each letter one step back: D → C, B → A, S → R.
- Pencil. We write in ink with a pen, and a pen is called "pencil".
What to do next
- Learn the 13 opposite pairs until you can say them in under 20 seconds.
- Solve 20 coding questions a day for a week, writing positions under every letter.
- Add one line to your mistake log for every wrong answer: which type it was, and what you missed.
- Practise alphabet questions alongside coding, because both use the same letter positions.
- Revise analogy, where letter analogies use these same shifts.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Staff Selection Commission website .
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