Coding-Decoding: Letter Shifts and Number Codes

In coding-decoding questions, a word or number is transformed by a hidden rule, and you must apply the same rule to another. Spotting the type of rule is half the job.

Know the alphabet positions

A	B	C	D	E	F	G	H	I	J	K	L	M
1	2	3	4	5	6	7	8	9	10	11	12	13

N	O	P	Q	R	S	T	U	V	W	X	Y	Z
14	15	16	17	18	19	20	21	22	23	24	25	26

The letters that are opposite each other (A–Z, B–Y, C–X, and so on) add up to 27.

Type 1: constant shift

Each letter moves forward or backward by a fixed number.

Example: CAT is coded as DBU. Each letter moves +1. So DOG becomes EPH.

Type 2: opposite letters

Each letter is replaced by its opposite: A ↔ Z, B ↔ Y, C ↔ X.

Example: CAT becomes XZG.

Type 3: reversed word

Example: If SMART is coded as TRAMS, then CLEAR becomes RAELC.

Type 4: alternating shifts

The first letter moves +1, the second −1, the third +1, and so on.

Example: CAT with +1, −1, +1: C→D, A→Z, T→U giving DZU.

Type 5: letters to numbers

Letters are replaced by their positions, sometimes with an operation.

Example: If A = 1, B = 2, then BAD = 2, 1, 4. If the code is the sum, BAD = 7.

Type 6: word-to-word substitution

Example: "sky" is called "sea", "sea" is called "tree". Where do fish live? Fish live in the sea, which is coded tree.

Number codes

Some questions give a few phrases with number codes and ask you to identify which number stands for which word, by comparing phrases that share one word and one number.

Method

1. Compare the given word and its code letter by letter.
2. Look for a consistent rule.
3. Test the rule on every letter.
4. Apply it to the new word.

Tips

• Write positions under letters to see shifts.
• Watch for the wraparound, where Z shifts to A.
• Try the simplest rule first.

Practice

Coding-decoding is easy to practise in short bursts.

Practise with Online Exam Question & Answer →