Classification (Odd One Out)

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Letter-cluster odd one out

⏱ 4 min read🧩 5 question types🎯 15 practice Q

The idea in one minute

Each option is a small letter cluster, like ACE or BDFH. Three clusters share an internal structure and one breaks it. Turn every letter into its alphabet position (A = 1 ... Z = 26) and write the steps between letters. The step tuple turns a feel question into simple arithmetic. If the tuples match, test vowels, opposite letters or position sums.

Letters are numbers

A = 1, B = 2 ... Z = 26. Learn the five EJOTY anchors: E = 5, J = 10, O = 15, T = 20, Y = 25. Every other letter is a short hop from one of them. R = T − 2 = 18.

Write the step tuple

For each cluster, write the jump from each letter to the next. That list is the step tuple.

ACE: A→C is +2, C→E is +2. Tuple (+2, +2).
Backward steps are negative: Z→X is −2.
After Z the count wraps to A: Y + 3 = B.

The tuple is the fingerprint of the cluster. Two clusters built by the same rule have the same tuple.

Tip: Write the positions under the letters of all four options in one pass. Then read off the tuples.

The rule families

Constant step. The same jump every time. DGJ, KNQ and PSV are all (+3, +3). TWY is (+3, +2), so TWY is odd.
Repeated mixed steps. The jumps differ but repeat across clusters. ACF, GIL and MOR are all (+2, +3). SUW is (+2, +2). With four letters, MPOR is (+3, −1, +3).
Opposite letters. A-Z, B-Y ... M-N. Their positions always add to 27. In DW, GT and KP the two letters add to 27. MO adds to 28, so MO is odd.
Vowel structure. NOP, HIJ and DEF carry their vowel in the middle (O, I, E). RST has no vowel at all.
Position property. The positions follow a number rule. B, H and L sit at even positions (2, 8, 12). G sits at 7, odd, so GLR is odd among even-position clusters.

Method in five steps

Write positions under every letter of all four options.
Write the step tuple of each option.
If three tuples match, the fourth option is the answer.
If all four tuples match, test vowels and opposite letters.
If all tuples differ, test position sums and even or odd positions.

Worked example

Odd one: ACF, GIL, MOR, SUW.

ACF: 1, 3, 6 → (+2, +3)
GIL: 7, 9, 12 → (+2, +3)
MOR: 13, 15, 18 → (+2, +3)
SUW: 19, 21, 23 → (+2, +2)

Answer: SUW.

Try the opposite family on its own. Odd one: DW, GT, KP, MO. D + W = 4 + 23 = 27. G + T = 7 + 20 = 27. K + P = 11 + 16 = 27. But M + O = 13 + 15 = 28. So MO is odd.

Example: Position-sum family. AMP = 1 + 13 + 16 = 30, DLN = 4 + 12 + 14 = 30, EKN = 5 + 11 + 14 = 30. BHS = 2 + 8 + 19 = 29. So BHS is odd.

The vowel note

Some papers print a note: "the odd one is not based on the number of consonants or vowels, or their position". That note removes the vowel family. Use steps, opposite letters or position sums instead.

Watch: Check every step of a tuple. Setters often spoil only the second or third step, so the first step alone is not enough.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

Constant step inside the cluster

How to spot it:

Every cluster has letters with equal gaps (ACE, DGJ, BFJN), forward or backward.

\text{step} = Q(\text{next}) - Q(\text{this}) \pmod{26}

Method

Convert letters to positions.
Write the step tuple for each cluster.
Three clusters have the same constant step; the fourth changes one step.

Why it works:

The setter copies one constant jump and spoils a single step in the odd cluster.

Try this

Find the odd one: DGJ, KNQ, PSV, TWY.

Show solution

D→G→J is (+3, +3). K→N→Q is (+3, +3). P→S→V is (+3, +3).
T→W is +3 but W→Y is +2. TWY is (+3, +2).
Rule: three clusters jump +3, +3; one does not.

Answer

TWY

Type 2very common2 practice Q

Mixed step tuple (same pattern repeated)

How to spot it:

Steps inside each cluster are unequal (+2, +3) or rise and fall (+3, −1, +3), but look similar across options.

Method

Write the full step tuple for each cluster.
Three tuples will be identical.
The cluster with a different tuple is odd.

Why it works:

The pattern is the tuple as a whole, not any single step.

Try this

Find the odd one: ACF, GIL, MOR, SUW.

Show solution

ACF: 1, 3, 6 gives (+2, +3).
GIL: 7, 9, 12 gives (+2, +3). MOR: 13, 15, 18 gives (+2, +3).
SUW: 19, 21, 23 gives (+2, +2).

Answer

SUW

Type 3common2 practice Q

Opposite-letter pairs (positions add to 27)

How to spot it:

Pairs like AZ, BY, DW, or clusters where one letter mirrors another from the other end of the alphabet.

Q(x) + Q(\bar{x}) = 27

Method

Add the positions of the two letters that should be opposite.
Opposite letters add to 27.
The cluster whose pair does not make 27 is odd.

Why it works:

A to Z and M to N all sum to 27, so one addition tests each option.

Try this

Find the odd one: DW, GT, KP, MO.

Show solution

D + W = 4 + 23 = 27. G + T = 7 + 20 = 27. K + P = 11 + 16 = 27.
M + O = 13 + 15 = 28, not 27.
Rule: three pairs are opposite letters, one is not.

Answer

Type 4common2 practice Q

Vowel position / vowel structure

How to spot it:

All clusters have similar steps, so steps cannot decide; vowels sit in a fixed slot in three clusters.

Method

Circle the vowels (A, E, I, O, U) in each cluster.
Note the number and position of vowels.
The cluster that differs is odd.

Why it works:

When step tuples are identical, the setter has used the vowels.

Try this

Find the odd one: NOP, HIJ, DEF, RST.

Show solution

NOP has O in the middle. HIJ has I in the middle. DEF has E in the middle.
RST contains no vowel at all.
Rule: three clusters carry a vowel in the middle slot.

Answer

RST

Type 5occasional2 practice Q

Position-number property (sum, even or odd positions)

How to spot it:

Step tuples are all different and there is no vowel pattern; the letters look random.

\sum Q(\text{letters}) = \text{constant}

Method

Write positions of all letters.
Test: equal position sums? All even or all odd positions? Squares?
The cluster that fails the common property is odd.

Why it works:

The setter chose letters to meet a number condition, not a step condition.

Try this

Find the odd one: AMP, DLN, EKN, BHS.

Show solution

AMP = 1 + 13 + 16 = 30. DLN = 4 + 12 + 14 = 30. EKN = 5 + 11 + 14 = 30.
BHS = 2 + 8 + 19 = 29.
Rule: three clusters sum to 30, one sums to 29.

Answer

BHS

Formula sheet

Step of a cluster

s_i = Q(L_{i+1}) - Q(L_i) \pmod{26}

Q is the alphabet position. Compute steps for every option; three must match as tuples.

Opposite-pair sum

Q(X) + Q(Y) = 27

Letter pairs whose positions add to 27: AZ, BY, ..., MN.

Shortcuts that save time

⚡ Step tuples in one line

Under each cluster write its steps: ACE → (+2, +2), BDF → (+2, +2), GIK → (+2, +2), MOP → (+2, +1). The mismatched tuple is the answer. No vowel counting is needed.

Example

Find the odd one: ACE, BDF, GIK, MOP.

Show solution

ACE: A→C +2, C→E +2. Tuple (+2, +2).
BDF and GIK also give (+2, +2).
MOP: M→O is +2 but O→P is +1. Tuple (+2, +1).

Answer

MOP

⚡ Vowel count as tiebreaker

If the step tuples agree, or the clusters look irregular, count the vowels (A, E, I, O, U). Three clusters with no vowel and one with a vowel (or the reverse) is a complete exam pattern.

Example

Find the odd one: BGT, CFD, HLM, APT.

Show solution

The step tuples are irregular, so steps cannot decide.
BGT, CFD and HLM contain no vowel.
APT contains the vowel A.

Answer

APT

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Computing steps without wrapping (Y to B read as −23).

Wrap past Z: Y + 3 = 25 + 3 = 28, then 28 − 26 = 2 = B.

Mistake 02

Forgetting U is a vowel.

The vowels are A, E, I, O, U. Circle them before counting.

Mistake 03

Checking only the first step of each cluster.

Write the full tuple. Setters often spoil the second or third step.

Mistake 04

Counting letters on fingers and slipping by one.

Use EJOTY anchors and subtract positions.

Mistake 05

Using the vowel family when the paper's note forbids it.

Read the note. When vowels are excluded, use steps, opposites or sums.

Quick revision

Read this the night before the exam.

EJOTY: E5, J10, O15, T20, Y25.
Step tuple = the fingerprint. Three tuples match, one differs.
Wrap past Z: above 26 subtract 26.
Opposite letters add to 27: AZ, BY ... MN.
Tuples all match? Test vowels next, then position sums and parity.
Vowel note printed? Vowels are excluded; use steps or sums.

Practice: 15 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 6 min · wrong answers go to your mistake notebook automatically.

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