In this guide
A letter series is a number series in disguise. Convert each letter to its position in the alphabet, find the pattern in the numbers, then convert the answer back into a letter. Once you know the positions by heart, these are among the fastest questions in the reasoning section.
NTPC asks letter series in a few shapes: single letters (B, E, H, K), letter pairs or groups (AZ, BY or BDF, HJL), and mixed letter–number terms (2B, 4D, 8H). The method is the same for all of them.
Letter 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 |
- EJOTY = 5, 10, 15, 20, 25. From any anchor, count one or two letters to reach the one you need.
- Opposite letters add up to 27: 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.
- Backward position = 27 − forward position. W is 23rd from the start and 4th from the end.
- Wrap-around: if a position goes past 26, subtract 26. Position 29 is C.
A method
- Write the position under every letter.
- Find the gaps between positions, exactly as in a number series.
- For groups, look inside each group (gaps between its letters) and across groups (how the first letter moves from group to group).
- Convert back and check the answer against every term.
Common patterns
| Pattern | Example | Rule |
|---|---|---|
| Constant gap forward | A, C, E, G | + 2 |
| Constant gap backward | Z, X, V, T | − 2 |
| Growing gap | A, B, D, G, K | + 1, + 2, + 3, + 4 |
| Pairs moving together | AB, DE, GH | Each pair + 3 |
| Opposite pairs | AZ, BY, CX | First + 1, second − 1 |
| Groups with inner gaps | BDF, HJL, NPR | Inner + 2; groups start + 6 |
| Two interleaved series | A, Z, C, X, E, V | Odd places + 2; even places − 2 |
| Letter with number | 2B, 4D, 8H | Number doubles; letter = number's position |
Worked questions
Q1. B, E, H, K, ?
Positions 2, 5, 8, 11: + 3 each time. Next is 14, which is N.
Q2. A, B, D, G, K, ?
Positions 1, 2, 4, 7, 11. Gaps + 1, + 2, + 3, + 4, so the next gap is + 5. 11 + 5 = 16, which is P.
Q3. AZ, BY, CX, ?
First letters: A, B, C, moving + 1. Second letters: Z, Y, X, moving − 1. Next pair: DW. Each pair is also an opposite pair (D + W = 4 + 23 = 27), which confirms it.
Q4. A, Z, C, X, E, V, ?
The series jumps between the two ends of the alphabet, so split it. Odd places: A, C, E (+ 2). Even places: Z, X, V (− 2). The missing term is the 7th, an odd place, so it is G.
Q5. BDF, HJL, NPR, ?
Inside each group the gap is + 2. The first letters are B (2), H (8), N (14), rising by 6. The next group starts at 20, which is T, and continues + 2: TVX.
Q6. W, Z, C, F, ?
Positions 23, 26, then 29 − 26 = 3 (C), then 6 (F). The rule is + 3 with wrap-around after Z. Next: 9, which is I.
Q7. 2B, 4D, 8H, ?
Numbers: 2, 4, 8, doubling, so the next is 16. Letters: B, D, H are positions 2, 4, 8, the same as the numbers. The 16th letter is P. Answer: 16P.
Continuous-pattern series
A related type shows a string with blanks, such as ab_ab_ab_, and asks which letters fill the blanks. Find the repeating block (here "ab" followed by one letter) and fill the blanks so the block repeats exactly. Write the string out in blocks of the repeating length; the answer is usually obvious once you do.
Common mistakes
- Counting on fingers. It is slow and error-prone. Use EJOTY anchors.
- Forgetting to wrap around after Z. Subtract 26 from any position above 26.
- Missing two series running together. If letters jump between the start and end of the alphabet, split odd and even places.
- Checking only inside a group and ignoring how the groups move, or the reverse.
- Off-by-one errors when moving backward. Z − 2 is X, not W.
Practice set
- D, G, J, M, ?
- Y, W, U, S, ?
- C, F, J, O, ?
- AZ, CX, EV, ?
- KM, LN, MO, ?
- X, A, D, G, ?
- B2, D4, F6, ?
- CEG, IKM, OQS, ?
Answers:
- P. Positions 4, 7, 10, 13: + 3, so 16.
- Q. Positions 25, 23, 21, 19: − 2, so 17.
- U. Positions 3, 6, 10, 15: gaps + 3, + 4, + 5, so the next is + 6 = 21.
- GT. First letters + 2 (A, C, E, G); second letters − 2 (Z, X, V, T).
- NP. Both letters + 1 each time; the gap inside each pair stays 2.
- J. + 3 with wrap-around: 24, then 27 − 26 = 1 (A), 4, 7, 10.
- H8. Letters + 2; the number is the letter's position.
- UWY. Inside each group + 2; group starts C (3), I (9), O (15) rise by 6, so the next starts at 21 (U).
What to do next
- Say the positions of ten random letters aloud each morning until you never count.
- Learn the 13 opposite pairs; they appear in series, analogies and coding.
- Solve 20 letter series, writing positions under the letters every time.
- Move on to coding–decoding, which uses the same shifts.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Railway Recruitment Boards website .
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