Letter and Symbol Series
Letter and symbol series questions test whether you can spot the rule that links one term in a sequence to the next, and use it to find the missing or next term. The PMDC MDCAT 2026 syllabus expects fluency with alphabet position rules, multiplicative and arithmetic patterns, and alternating sequences. Expect 1-2 quick-scoring MCQs from this topic.
The alphabet position table
The single most useful tool for this topic is the alphabet-position table. Memorise the forward positions; the reverse positions are simply 27 minus the forward number.
| Letter | A | B | C | D | E | F | G | H | I | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Forward | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| Reverse | 26 | 25 | 24 | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 |
| Letter | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Forward | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 |
| Reverse | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
Anchor letters: E=5, J=10, O=15, T=20, Y=25 — the every-fifth-letter checkpoints. Memorise them and count from the nearest anchor.
Common pattern types — recognise at a glance
| Pattern type | What to check | Example |
|---|---|---|
| Constant addition | Differences are equal | A, C, E, G, I (+2) |
| Increasing addition | Differences themselves increase | A, B, D, G, K, P (+1, +2, +3, +4, +5) |
| Constant subtraction | Backwards equal step | Z, X, V, T, R (−2) |
| Geometric (doubling) | Each term × constant ratio | 1, 2, 4, 8, 16 stars |
| Multiplicative letter jumps | Gap itself multiplies | A, B, D, H, P, F (gaps +1,+2,+4,+8,+16 with wrap) |
| Alternating sub-series | Split odd/even positions | A, Z, B, Y, C, X, D |
| Letter-pair / triple | Treat each position separately | AZ, CY, EX, GW, IV |
| Wrap-around | Pass Z → back to A | X, Y, Z, A, B |
Arithmetical Operations
Many series advance the alphabet position by a fixed difference. Convert each letter to its number, look at the differences, and read off the rule.
Each letter is shifted forward by the same fixed amount. Example: A, C, E, G, ? Differences: +2, +2, +2 — next letter is I. Translate to numbers: 1, 3, 5, 7, 9.
The shift itself grows. Example: A, B, D, G, K, ? Differences: +1, +2, +3, +4, +5 — next is K + 5 = P. As numbers: 1, 2, 4, 7, 11, 16.
The series moves backwards through the alphabet. Example: Z, X, V, T, ? Differences: -2, -2, -2 — next is R.
If a forward step exceeds Z, wrap back to A and continue. Treat the alphabet as a 26-cell ring. Example: X, Y, Z, A, B, C — the series simply continues past Z by returning to A.
Worked example — arithmetic letter series
Series: C, F, I, L, ? Step 1: Convert — 3, 6, 9, 12. Step 2: Difference is +3 throughout. Step 3: Next number is 12 + 3 = 15 → O.
Geometrical Progression
Geometric series multiply (or divide) by a fixed ratio. They appear most often with symbol counts — e.g., the number of dots, dashes, or stars in each term doubles or halves — but they can also drive the alphabet jumps.
Example: *, **, ****, ********, ? Each term has twice as many stars as the previous — next is sixteen stars. Symbol counts: 1, 2, 4, 8, 16.
Example: 64, 32, 16, 8, ? Each term is half the previous — next is 4. Useful when symbol counts shrink.
Example: A, B, D, H, P, ? Differences: +1, +2, +4, +8 (each gap doubles). Next gap is +16; P is 16 → 16 + 16 = 32. 32 - 26 = 6, so wrap to F. Next term is F.
Spotting an arithmetic vs a geometric pattern
Compute the differences first. If the differences are equal, the series is arithmetic. If the ratios are equal, the series is geometric. If neither, look for an alternating pattern next.
Sequential Orders
"Sequential order" series interleave two or more sub-series. The trick is to split the terms into odd-positioned and even-positioned groups and analyse each group separately.
Example: A, Z, B, Y, C, X, ? Odd positions: A, B, C (forward by 1). Even positions: Z, Y, X (backward by 1). Next term is at position 7 (odd) — D.
Example: A, C, F, J, O, ? Differences: +2, +3, +4, +5. Next gap is +6; O is 15 → 21 → U.
Each term is a pair (or triple) of letters. Treat each position separately. Example: AZ, CY, EX, GW, ? First letters: A, C, E, G — +2 each. Second letters: Z, Y, X, W — -1 each. Next pair: I, V → IV.
Example: B, D, G, K, P, ? Skips: 2, 3, 4, 5 (to count letters jumped). Next skip: 6. P (16) + 6 = 22 → V.
Worked MCQs
Five MCQs that capture the high-yield testing patterns for letter and symbol series. Read the explanation even when you get the answer right — it's where the deeper concept lives.
Q1. Find the next term in the series: B, E, H, K, ?
Convert: 2, 5, 8, 11. Differences: +3, +3, +3. Next number = 11 + 3 = 14 → N. Constant-addition series.
Q2. Find the next term: A, C, F, J, ?
Convert: 1, 3, 6, 10. Differences: +2, +3, +4. Next difference is +5. 10 + 5 = 15 → O. The triangular-number pattern.
Q3. Find the next term: AZ, BY, CX, DW, ?
First letters advance A, B, C, D — next is E. Second letters retreat Z, Y, X, W — next is V. Pair = EV.
Q4. Find the next term: Z, W, T, Q, ?
Convert: 26, 23, 20, 17. Differences: -3 each step. 17 - 3 = 14 → N. A descending arithmetic series.
Q5. Find the next term: A, B, D, H, P, ?
Convert: 1, 2, 4, 8, 16. Each gap doubles (+1, +2, +4, +8). Next gap = +16. 16 + 16 = 32; subtract 26 to wrap = 6 → F. A geometric progression in the gaps.
Quick Recap
- Memorise alphabet positions; anchor on E=5, J=10, O=15, T=20, Y=25.
- Arithmetic series: equal differences. Geometric series: equal ratios.
- If a step takes you past Z, wrap around to A.
- For paired or triple letters, treat each position as its own sub-series.
- Mixed series: split into odd/even positions and analyse separately.
- Use C-D-S: Convert, Differences, Split.