Coding and Decoding turns a known word into a secret pattern, and your job in the AFCAT is to spot the rule and reverse it. It is one of the highest-scoring and fastest Reasoning topics – pure logic, no formulas to memorise. If you know your alphabet positions cold, most questions fall in under 30 seconds. This guide trains exactly that speed.
Why Coding-Decoding is a guaranteed scorer
In every AFCAT Reasoning and Military Aptitude paper you can expect 3 to 5 coding-decoding questions. They need no external knowledge – only a clear head and quick alphabet sense. That makes them ideal for protecting your score against the negative marking that punishes guesswork elsewhere. Where a current-affairs or vocabulary question can defeat even a prepared candidate, a coding question never can: every clue you need is printed in the question itself.
The whole topic rests on one idea: a fixed rule converts the original (called the plain word) into a code. Find the rule from the example given, then apply the same rule – forward to encode, backward to decode. Because the AFCAT is a speed test (roughly one minute per Reasoning question), the candidate who recognises the rule fastest, not the one who knows the most, wins these marks.
Across years of papers, the Air Force tends to recycle a handful of patterns. Once you have drilled the six families covered below – letter-shift, opposite-letter, letter-to-number, mixed-operation number, symbol and word-substitution codes – you will recognise almost every variant on sight. The rest of this guide builds that recognition step by step.
Coding never changes randomly. The same letter or position is always treated the same way throughout a question. Spot the pattern once and the rest is mechanical.
The foundation: alphabet position numbers
Almost every code uses the position of a letter in the alphabet. You must recall these instantly, both forward and backward.
Forward positions: A=1, B=2, C=3 … Z=26.
Reverse positions: Z=1, Y=2, X=3 … A=26. The reverse value is found by the rule 27 − forward position.
Memorise five anchor letters: E=5, J=10, O=15, T=20, Y=25. This is the famous EJOTY word. From any anchor you count just one or two steps to the letter you need – far faster than reciting A,B,C… from the start.
Example with EJOTY: to find the position of R, start at T=20 and step back two letters (T→S→R), giving R=18. To find L, start at J=10 and step forward two (J→K→L), giving L=12. With practice you will jump to any letter using at most two or three steps from an anchor, which is the single biggest time-saver in this whole topic.
It also helps to know the alphabet split into halves. The first half runs A–M (1–13) and the second half runs N–Z (14–26). If a letter sits in the second half you can confidently use the nearest anchor (O, T or Y) rather than counting all the way from A. Spend a few minutes each morning reciting positions until they become automatic; this single habit separates fast solvers from slow ones.
Type 1: Letter-shift (Caesar) coding
Here each letter moves forward or backward by a fixed number of steps.
If CAT is coded as DBU, each letter has moved +1 (C→D, A→B, T→U). To decode, move −1.
The shift can be +2, +3, −1, and so on. Always confirm the shift on two or three letters before trusting it – one matching letter could be a coincidence. A common AFCAT twist is an alternating shift, where odd-position letters move +1 and even-position letters move −1 (or +2 and +3). If a single uniform shift does not fit all letters, suspect an alternating pattern and test the odd and even positions separately.
Another variant places the shift on the whole word reversed: the letters are first written backwards and then shifted, or shifted and then reversed. When the code letters seem related but appear in the wrong order, quickly check whether reversing the word makes the shift obvious. These reversed-shift codes look intimidating but collapse instantly once you spot the reversal.
When a forward shift pushes past Z, wrap around to A. For example, Y with a +3 shift gives Y→Z→A→B = B. Treat the alphabet as a circle.
Type 2: Opposite-letter (reverse alphabet) coding
Each letter is replaced by its opposite from the other end of the alphabet. A↔Z, B↔Y, C↔X, and so on. The pair always adds up to 27.
Opposite of a letter = letter at position (27 − its position). So D (4) becomes W (23), because 27 − 4 = 23.
Useful opposite pairs worth memorising: A↔Z, B↔Y, C↔X, D↔W, E↔V, M↔N. Note that M and N are the middle pair and swap with each other. Because the opposite of an opposite returns the original letter, the same rule both encodes and decodes – you never have to reverse direction, which makes this the friendliest code in the exam.
The opposite-letter code is also frequently combined with a shift. For example, a question may first take the opposite letter and then move it forward by one. If a pure opposite mapping is off by a constant amount on every letter, you are looking at this opposite-plus-shift hybrid; subtract the constant first, then apply the 27-minus rule.
If a coded word looks like gibberish but is the same length as the original, test the opposite-letter rule first – AFCAT loves it.
Type 3: Letter-to-number coding
Letters are replaced by numbers. The number is usually the letter’s position, or position with a small operation (add, subtract, multiply, or use the reverse position).
If BAD is coded as 2 1 4, the rule is simply forward position. If it were 4 2 8, the rule is position × 2.
When the numbers are larger than 26, suspect operations like position + a constant, position × a factor, or the sum/product of two letters. Check the smallest letter first to spot the operation quickly.
Sometimes the whole word maps to a single number – often the sum of all letter positions. For CAB: 3 + 1 + 2 = 6. Other single-number rules include the product of positions, the count of letters multiplied by a fixed value, or the sum of only the vowels or only the consonants. When a word reduces to one number, list the position sum first because it is the most common rule by far.
Reverse-position number codes are also popular. Here A=26, B=25, down to Z=1. If a small letter like A maps to a large number such as 26, you are almost certainly looking at the reverse scale rather than a multiplication. Always test both the forward and reverse position scales before assuming an arithmetic operation, as this quick check saves you from chasing a pattern that is not there.
Type 4: Symbol and substitution coding
Symbol coding swaps letters or words for symbols (+, ×, ▲, ●). Substitution coding replaces whole words with other words – for example, in a code language ‘sky is blue’ becomes ‘ka na pa’.
For substitution, compare two statements that share a common word. The shared word maps to the shared code; isolate it, then deduce the rest by elimination.
Lay the statements one under the other. Strike out matched word-code pairs. Whatever remains uniquely on each side must correspond.
Worked logic for substitution: suppose ‘la pa ni’ means ‘he is tall’ and ‘pa sa ru’ means ‘is she clever’. The common English word is ‘is’ and the common code is ‘pa’, so ‘is’ = pa. From the first statement the remaining codes la and ni stand for ‘he’ and ‘tall’ in some order; a third statement is usually supplied to pin them down exactly.
In symbol coding, never assume + means add or × means multiply – the symbols are arbitrary labels. Treat them as nonsense tokens and match purely by the rule the question states.
Worked example: cracking a shift code
Let us solve a typical letter-shift question step by step.
If ‘TIGER’ is coded as ‘WLJHU’, how is ‘LION’ coded?
Answer: OLRQ. Total time with EJOTY anchors – about 20 seconds.
Speed shortcuts that save the clock
These tricks turn a 60-second question into a 20-second one.
- 27-minus rule: for any opposite-letter code, subtract the position from 27 instead of counting backwards.
- EJOTY jumps: never recite the alphabet from A. Jump from the nearest anchor (E, J, O, T, Y).
- Letter gap, not position: for shift codes you only need the gap between letters, so even rough position sense works.
- Length match: if code length equals word length, the rule is one-letter-for-one-letter (shift, reverse, or number) – rule out word substitution.
Write the shift number above the first letter as soon as you find it. This stops you re-deriving it for every remaining letter under exam stress.
Common mistakes and traps
Confusing forward and reverse positions. Re-read whether the code counts from A or from Z before you start.
Forgetting to wrap around at Z. A +4 shift on X is not a dead end – X→Y→Z→A→B = B.
Trusting a pattern after checking only one letter. Always verify on at least two letters, because shift and opposite rules can match on a single coincidence.
In encode questions you move the rule forward; in decode questions you reverse it. Read the question stem carefully – mixing the direction is the single biggest avoidable error.
Previous-year style question
Q. In a certain code, ‘ROSE’ is written as ‘ILHV’. How will ‘TREE’ be written in that code?
Answer: GIVV. The rule is the opposite-letter code (27 − position): R(18)→I(9), O(15)→L(12), S(19)→H(8), E(5)→V(22). Apply to TREE: T(20)→G(7), R(18)→I(9), E(5)→V(22), E(5)→V(22), giving GIVV.
Notice how recognising the opposite-letter pattern from a single pair (E→V, since 5 + 22 = 27) instantly identified the rule.
How to practise for full marks
Coding-decoding rewards drill, not theory. A few minutes daily builds the reflexes you need on exam day. Reading about the rules is not enough; the speed comes only from repeated hands-on solving until position recall and rule-spotting become automatic.
Have a fixed order of attack for every question. First count the letters and compare lengths to decide the family. Next test the easy rules in order – uniform shift, opposite letter, forward position number – before trying anything exotic. Verify on two letters, then apply. This disciplined sequence stops you from staring blankly and keeps each question under the one-minute budget.
- Write A–Z with positions once a day until you can recall any position in under two seconds.
- Solve 10 mixed-type questions daily and time yourself – aim for under 30 seconds each.
- Keep an error log: note whether the slip was direction, wrap-around, or position recall.
- Revisit AFCAT PYQs to internalise the five recurring types in this guide.
- Find the rule from the example, then reverse it to decode.
- Forward A=1…Z=26; reverse Z=1, with opposite pairs summing to 27.
- Use EJOTY anchors (E5, J10, O15, T20, Y25) to skip counting.
- Always wrap around at Z and verify the rule on two letters.
- Match word length first to choose between shift, reverse, number or substitution codes.
Frequently asked questions
How many coding-decoding questions come in AFCAT?
Typically 3 to 5 questions per paper in the Reasoning and Military Aptitude section. Because they need no outside knowledge, they are among the safest marks to target.
What is the EJOTY trick?
EJOTY is a memory aid for alphabet positions: E=5, J=10, O=15, T=20, Y=25. From any of these anchors you count just a step or two to the letter you need, avoiding a full A-to-Z recital.
How do I spot the opposite-letter code quickly?
Check if a letter and its code add up to 27 in alphabet position (for example E=5 and V=22). If yes, the whole word uses the reverse-alphabet rule.
What is the most common mistake in this topic?
Mixing up encode and decode direction, and forgetting to wrap around at Z. Read the stem to see which way the rule goes, and treat the alphabet as a circle.
Do I need to memorise the whole alphabet with positions?
Yes, fluency with forward and reverse positions is essential. With daily practice and the EJOTY anchors you will recall any position in seconds, which is what makes this topic so fast to score.
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