The alphabet positions to know
A = 1, B = 2, … M = 13, N = 14, … Z = 26. Remember the pairs that add to 27: 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. These are the "opposite" letters used in reverse-alphabet codes.
Common types of rules
| Type | Example rule |
|---|---|
| Fixed shift | Each letter moves forward by 1 (A→B) |
| Changing shift | +1, +2, +3 and so on |
| Reverse alphabet | A↔Z, B↔Y |
| Letter to number | A = 1, B = 2, so CAB = 312 |
| Word substitution | "Apple" is called "mango" |
| Symbol or number groups | Match the words across sentences to find each code |
| Rearrangement | Letters reversed or swapped in pairs |
A method
- Write the positions of the given word's letters and of its code.
- Compare letter by letter: a constant shift, a reverse or a changing shift.
- Check the rule on every letter of the example.
- Apply the rule to the new word.
Worked examples
Example 1. If COLD is coded as DPME, how is WARM coded? Each letter moves one place forward: W→X, A→B, R→S, M→N, so XBSN.
Example 2 (reverse alphabet). If CAT is coded as XZG, how is DOG coded? C(3)→X(24): 27 − 3 = 24 ✓. A(1)→Z(26) ✓; T(20)→G(7) ✓. So D(4)→W(23), O(15)→L(12), G(7)→T(20): WLT.
Example 3 (changing shift). If BAKE is coded as CCNI, how is FAME coded? First find the rule: B(2)→C(3): +1; A(1)→C(3): +2; K(11)→N(14): +3; E(5)→I(9): +4. So the shift increases by 1 each time. For FAME: F(6)+1 = G; A(1)+2 = C; M(13)+3 = P; E(5)+4 = I: GCPI.
Example 4 (word codes). In a code, "red blue sky" is "ka mi do", "blue sea" is "mi pa" and "sky high" is "do la". Common word "blue" ↔ "mi"; common word "sky" ↔ "do". So "red" is "ka".
Example 5 (number codes). If A = 1, B = 2 and so on, the code for "BAD" is 214.
Example 6 (sum of positions). The sum of positions of the letters in "CAB" is 3 + 1 + 2 = 6.
Common traps
- Assuming the first shift is constant without checking later letters.
- Counting positions wrongly beyond M (the halfway point).
- Wrapping around the alphabet: Z + 1 = A.
- Missing the position-dependent rule where the shift changes with place.
Speed tips
- Memorise the letter positions and the 27-pairs.
- Write numbers above the letters.
- For word codes, compare sentences that share exactly one word.
A practice routine
- Day 1: fixed shift and reverse alphabet, ten questions.
- Day 2: changing shifts and positions.
- Day 3: word and number codes.
- Day 4: a timed set of 12 at 45 seconds each.
Where it appears
Coding-decoding appears in banking reasoning, SSC CGL reasoning, railway reasoning and MAT papers. Related topics: number series and blood relations.
Practice tests below are published by institutes and educators on TestMint.
