Coding-Decoding
Outline & Learning Goals
By the end of this chapter, you will be able to:
- Understand what coding-decoding questions actually test.
- Identify the 5 main types of coding patterns asked in SSC, Banking, Railway and other exams.
- Crack any letter-coding question in under 30 seconds using a fixed method.
- Spot and avoid the examiner's favourite traps.
- Build your own "code-breaking habit" through active recall.
1. The Hook: You Are Already a Code-Breaker
Have you ever written "ILU" instead of "I Love You"? Or read a friend's message like "gr8" and instantly understood "great"?
Congratulations — you have been doing coding-decoding your whole life. The examiner is not testing anything new. He is only testing whether you can find a hidden rule quickly and apply it without panic.
That is the whole game: Find the rule. Apply the rule. Move on.
2. The Common Struggle (And Why It Happens)
Most students open a coding-decoding question, stare at CAT = 3120 and think: *"Where do I even start?"*
The struggle is real, and it has one root cause: students try to guess the rule randomly instead of checking patterns in a fixed order.
Random guessing = wasted minutes.
Fixed order = answer in seconds.
This chapter gives you that fixed order.
3. What Is Coding-Decoding?
- Coding = converting a word/message into another form using a rule (letters → letters, letters → numbers, words → words).
- Decoding = reversing the process — reading the code and finding the original message or the rule.
In exams, you are usually given one or two examples of coded words, and then asked to code (or decode) a new word using the same rule.
> Golden Line: The rule used in the example is the rule for your answer. Your only job is to *discover* it.
🧠 Active Recall: In your own words — what is the difference between coding and decoding? Say it aloud before reading further.
4. The Alphabet Numbers — Your Only Weapon
Every coding question stands on one foundation: letter positions.
A=1, B=2, C=3 … Z=26.
Mnemonic Trick: "EJOTY"
Remember the positions of E, J, O, T, Y — they are multiples of 5:
| Letter | E | J | O | T | Y |
|---|---|---|---|---|---|
| Position | 5 | 10 | 15 | 20 | 25 |
Now any letter is just ±1, ±2 from these anchors. Want R? T is 20, so S=19, R=18. Done in 2 seconds — no counting from A.
Reverse Positions (Opposite Letters)
Pairs that add to 27 are called opposite letters:
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.
Mnemonic: "AZad BoY CX DWar EVen FUll GT HS IRon JQ KP LOve MN" — or simply remember: *position + opposite position = 27.*
🧠 Active Recall: Close the book. Write EJOTY with positions. Write the opposite of H, M, C. Check yourself. (Answers: H↔S, M↔N, C↔X.)
5. The Five Types of Coding (The Fixed Order)
When you see a question, check these types in this order:
Type 1: Letter Shifting (+/− fixed number)
Each letter moves forward or backward by a fixed number.
Example: If CAT is coded as DBU, how is DOG coded?
- C+1=D, A+1=B, T+1=U → rule is +1.
- So DOG → EPH. ✅
Variation: shifting increases: +1, +2, +3… (e.g., A→B, B→D, C→F).
Type 2: Letter-to-Number Coding
Letters are replaced by their positions (or a twist on positions).
Example: If CAT = 3-1-20, then DOG = 4-15-7.
Twist: sometimes the code is the sum of positions: CAT = 3+1+20 = 24.
Another twist: reverse positions (A=26 … Z=1).
Type 3: Opposite Letter Coding
Each letter is replaced by its opposite (sum = 27).
Example: If HIGH is coded as SRTS, check: H↔S, I↔R, G↔T, H↔S. ✅ Rule = opposite letters.
Type 4: Word Rearrangement / Jumbling
Letters stay the same but change order — first↔last, or the word is written in reverse.
Example: If TABLE is coded as ELBAT, the rule is reverse the word. So CHAIR → RIAHC.
Type 5: Substitution Coding (Word → Word)
One word stands for another: "red is called blue, blue is called green…"
Example: If "sky is called water", "water is called road", then where does a fish swim? In water — but water is called road. Answer: road.
⚠️ In this type, the answer is always the code name, not the real thing.
🧠 Active Recall: Name the 5 types in order without looking. Which type uses the number 27?
6. The 30-Second Solving Method (Step-by-Step)
For every question, follow these 4 steps:
Step 1 — Write positions. Put the alphabet number above each letter of the given word and its code.
Step 2 — Compare letter by letter. Is it +1? −2? Opposite (sum 27)? Reversed?
Step 3 — Confirm with the second letter. Never decide the rule from one letter alone — one letter can fit many rules by accident.
Step 4 — Apply to the new word and match with the options.
Worked Example: If PEN is coded as QFO, code for INK?
- P(16)→Q(17): +1. E(5)→F(6): +1. N(14)→O(15): +1. Rule confirmed: +1.
- I→J, N→O, K→L → JOL. ✅
🧠 Active Recall: Why is Step 3 (confirming with a second letter) so important? Think of an example where +1 and "opposite" could look the same for one letter.
7. A Slightly Harder Pattern (Intermediate Level)
Increasing shift: If MAN is coded as NDP, find the rule.
- M(13)→N(14): +1. A(1)→D(4): +3? That looks odd — recheck. A→D is +3, but wait: the examiner often uses +1, +2, +3. Check again: M→N = +1, A→C would be +2, but the code has D. So compute each letter separately: M→N = +1, A→D = +3, N→P = +2. No clean pattern — which means we should suspect a different rule, such as reverse positions or rearrangement, before forcing a shift rule.
The lesson: always compute each letter separately and look for a clean pattern. If the pattern looks messy, either you mis-added (recheck positions first) or the rule belongs to another type.
Clean example of increasing shift: If CAT is coded as DCW — C+1=D, A+2=C? No: A+2=C, but the code's second letter is C only if… compute carefully: C(3)+1=D(4), A(1)+2=C(3), T(20)+3=W(23). So DCW fits +1, +2, +3 perfectly. ✅
Number-sum twist: If GO = 22 (G=7, O=15; 7+15=22), then DO = 4+15 = 19.
8. Summary Table
| Type | Rule | Quick Signal |
|---|---|---|
| Letter shifting | +n or −n for every letter | Code letters are "neighbours" of original |
| Letter-to-number | Position value / sum of positions | Code is made of numbers |
| Opposite letters | Position + code position = 27 | A↔Z, M↔N type pairs |
| Rearrangement | Reverse or swap letters | Same letters, different order |
| Substitution | One word "called" another | "is called / means" in the question |
Solving mantra: Positions → Compare → Confirm → Apply.
⚠️ Caution: Examiner's Trap
Trap 1 — More than one possible rule in the options.
A word coded as +1 might *also* look like a reverse-position code for one letter. The examiner designs options so that a half-checked rule gives a wrong-but-tempting answer. Defence: always confirm the rule on at least two letters before touching the options.
Trap 2 — Difficult/unusual words to scare you.
Words like "ZEPHYR" or "OBTUSE" are used to create panic — but the rule is still just +1 or opposite letters. Defence: ignore the meaning of the word completely. It is only a packet of letters.
Trap 3 — Double-relationship questions.
In substitution chains ("A is called B, B is called C"), students answer B when the correct answer is C, because they stop one step early. Defence: read the question twice — *what is actually being asked*, and follow the chain to the last step.
📌 Review Reminder
Review this chapter after 2 days. Redo the worked examples with a pen, recite EJOTY and the opposite-letter pairs from memory, and test yourself on the 5 types in order. Ten minutes of recall now will save you ten marks later.