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Classification (Odd One Out)

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high importance~3 Q in Tier 19 formulas⚡ 9 shortcuts6 subtopics
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Subtopic 4 of 6·← Number odd one out·Pair odd one out →

Letter-cluster odd one out

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Each option is a small letter cluster; three share an internal structure and one breaks it. Read each cluster as steps between its letters (A=1...Z=26, wrap around). The structure bank:

  1. Constant step: ACE (+2), BDF (+2), GIK (+2).
  2. Mixed constant steps: ABD (+1, +2).
  3. Vowel patterns: clusters with exactly one vowel vs none vs all vowels.
  4. Opposite pairs: AZ, BY, CX (positions sum to 27).
  5. Consecutive runs: ABC, KLM, STU.
  6. Pair structures: pairs where second letter = first + k.

Compute steps for all four options in one pass; the odd cluster's step tuple differs. This converts a 'feel' question into arithmetic.

📖 Notes🧩 5 question types⚡ Formulas🎯 15 practice Q

Detailed notes

What is letter-cluster classification?

Each option is a small group of letters, such as ACE or BDFH. Three groups are built by the same rule; one is not. The rule is almost always about positions in the alphabet, so first turn letters into numbers.

Tool 1: alphabet positions

A = 1, B = 2, ... Z = 26. Remember EJOTY: E = 5, J = 10, O = 15, T = 20, Y = 25. Any letter is at most two steps from one of these. Example: R is T − 2 = 18.

Tool 2: steps inside a cluster

Write the jump from each letter to the next. ACE: A→C is +2, C→E is +2, so the step tuple is (+2, +2). After Z the count wraps back to A (Y + 3 = B). Backward steps are negative (Z→X is −2).

The rule families

1. Constant step. Same jump every time. DGJ (+3, +3), KNQ (+3, +3), PSV (+3, +3); TWY is (+3, +2) → odd.

2. Mixed but repeated steps. The jumps are not equal but the same pattern repeats in every cluster. ACF (+2, +3), GIL (+2, +3), MOR (+2, +3); SUW (+2, +2) → odd. For four letters: MPOR is (+3, −1, +3).

3. Opposite letters. Letters that sit at the same distance from both ends: 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. Their positions always add up to 27. In DW, GT, KP each pair adds to 27; MO adds to 28 → odd.

4. Vowel structure. A, E, I, O, U are vowels. Three clusters may have a vowel in the same place (NOP, HIJ, DEF have the vowel in the middle) while one has none (RST).

5. Position property. The positions follow a number rule: sum of positions is the same (AMP = 1 + 13 + 16 = 30), or all positions are even (B, H, L = 2, 8, 12).

Method in five steps

  1. Write positions under every letter of all four options.
  2. Write the step tuple of each option.
  3. If three tuples match, the fourth option is the answer.
  4. If all four tuples match, test vowels and opposite pairs.
  5. If tuples are all different, test position sums and even/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.

Exam note

Some papers add a note such as "the odd one is not based on the number of consonants/vowels or their position in the cluster". That note simply removes the vowel family - use steps or opposite letters instead.

Common mistakes

  • Counting letters on fingers and slipping by one. Use EJOTY and subtract positions.
  • Forgetting the wrap after Z (X + 4 = B).
  • Checking only the first step of each cluster; the second or third step is often where the odd cluster breaks.

Quick revision

RuleCheckExample of odd one
Constant stepAll jumps equalTWY among DGJ, KNQ, PSV
Repeated mixed stepsSame tupleSUW among ACF, GIL, MOR
Opposite lettersPositions add to 27MO among DW, GT, KP
Vowel placeVowel in same slotRST among NOP, HIJ, DEF
Position sumSame totalBHS (29) among sums of 30
Even positionsAll letters evenGLR among BHL, DJP, FNT

Types of questions asked

Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.

Type 1: Constant step inside the clustervery common3 practice Q
How to spot it:

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

step=Q(next)−Q(this)(mod26)\text{step} = Q(\text{next}) - Q(\text{this}) \pmod{26}
  1. Convert letters to positions.
  2. Write the step tuple for each cluster.
  3. Three clusters have equal steps with the same value; the fourth changes one step.

Why it works: the setter copies one constant jump and spoils one step in the odd cluster.

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

Steps: (+3, +3), (+3, +3), (+3, +3), (+3, +2). Answer: TWY.

Type 2: Mixed step tuple (same pattern repeated)very common2 practice Q
How to spot it:

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

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

Why it works: the pattern is the tuple as a whole, not any single step.

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

Tuples: (+2, +3) three times, SUW is (+2, +2). Answer: SUW.

Type 3: Opposite-letter pairs (position sum 27)common2 practice Q
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(xˉ)=27Q(x) + Q(\bar{x}) = 27
  1. Add the positions of the two letters that should be opposite.
  2. Opposite letters add to 27.
  3. The cluster whose pair does not make 27 is odd.

Why it works: A↔Z, B↔Y ... M↔N all sum to 27, so one addition tests each option.

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

D + W = 4 + 23 = 27, G + T = 27, K + P = 27, M + O = 13 + 15 = 28. Answer: MO.

Type 4: Vowel position / vowel structurecommon2 practice Q
How to spot it:

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

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

Why it works: when step tuples are identical, the setter has used vowels.

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

NOP, HIJ, DEF each have a vowel in the middle (O, I, E). RST has no vowel. Answer: RST.

Type 5: Position-number property (sum, even/odd positions)occasional2 practice Q
How to spot it:

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

∑Q(letters)=constant\sum Q(\text{letters}) = \text{constant}
  1. Write positions of all letters.
  2. Test: equal position sums? all even or all odd positions? squares?
  3. 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.

Example: Find the odd one: AMP, DGT, EKN, BHS.

Sums: 1+13+16 = 30, 4+7+19 = 30, 5+11+14 = 30, 2+8+19 = 29. Answer: BHS.

Formulas

Step of a cluster
si=Q(Li+1)−Q(Li)(mod26)s_i = Q(L_{i+1}) - Q(L_i) \pmod{26}

Compute steps for every option; three must match as tuples.

Opposite-pair sum
Q(X)+Q(Y)=27Q(X) + Q(Y) = 27

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

Shortcut tricks

⚡ 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 naming of vowels needed.

Example: Q. ACE, BDF, GIK, MOP

Sol. ACE (+2,+2), BDF (+2,+2), GIK (+2,+2), MOP (+2,+1) → MOP.

Steps make letter classification as objective as number classification.

⚡ Vowel count as tiebreaker

If step tuples coincide or the clusters differ in length, count vowels (A, E, I, O, U). Three clusters with exactly one vowel and one with none (or two) is a complete CGL pattern.

Example: Q. BGT, CFD, HLM, APT

Sol. Vowel counts: 0, 0, 0, 1 (A) → APT. (Steps here don't separate the options cleanly, so the vowel level decides.)

Circle vowels first when clusters look irregular.

Where students lose marks

  • Computing steps without wrapping (Y to B is +3, not −23).

  • Forgetting U is a vowel; 'QST' has no vowel even though Q looks vowel-ish next to U.

  • Reading pairs left-to-right when the structure is right-to-left (ZYX, Wax...); check both directions before concluding.

Practice sets — 15 questions

Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.

Topic test · 10 questions

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