Letter and Alphanumeric Series

Letter and alphanumeric series test the same pattern-spotting as number series, but on the alphabet and mixed strings. They appear in TCS and Cognizant reasoning sets. The one prerequisite is knowing each letter's position.

Essentials

• Letter positions: A=1, B=2, … Z=26 (and reverse: Z=1 from the back, useful as "27 − position").
• Letter series often skip a fixed number of letters (A, C, E, G → +2).
• Alphanumeric series mix letters, digits and symbols; typical asks: nth from left/right, "how many letters are immediately followed by a digit", elements between two markers.

Item	Value / rule
A … Z	1 … 26
Opposite of a letter	27 − its position
Gap of +2	A, C, E, G, I
nth from right	(total − n + 1) from left

Example: A, C, F, J, O → gaps +2, +3, +4, +5 → next +6 = U. ✓ (O=15, +6 → 21 = U.)

Exam Tricks & Tips

• 🎯 Memorise letter positions in blocks of five (A-E =1-5, F-J =6-10, …) for instant lookup.
• 🎯 The opposite letter is 27 − position (A↔Z, B↔Y, …).
• 🎯 For alphanumeric "nth from right", convert to (total − n + 1) from the left.
• 🎯 "Immediately followed/preceded by" — read the string one step at a time; don't skip.
• 🎯 EJOTY (5,10,15,20,25) is a handy anchor for quick position estimates.
• ❌ Don't miscount from the wrong end — always confirm whether it's from the left or the right.

Expected exam pattern: next letter/group, nth element from an end, and counting elements meeting a condition. 1 to 2 questions, ~45 seconds.

Quick recap: know positions (and 27−position for opposites); spot the letter gap; convert right-counts to left-counts; read alphanumeric strings carefully.

Q1. Position of the letter H?
A1. 8.

Q2. Opposite letter of C?
A2. 27 − 3 = 24 → X.

Q3. Next letter: A, C, E, G, …?
A3. I (gap +2).

Q4. Next in A, C, F, J, O?
A4. U (gaps +2,+3,+4,+5,+6).

Q5. nth from right equals what position from left?
A5. (total − n + 1).

Q6. EJOTY stands for which positions?
A6. 5, 10, 15, 20, 25.

Q7. Position of the letter T?
A7. 20.

Q8. Opposite of the letter J?
A8. 27 − 10 = 17 → Q.

Q9. In a 15-symbol string, 4th from the right is which from left?
A9. 15 − 4 + 1 = 12th.

Q10. Next letter: B, D, G, K, …?
A10. P (gaps +2,+3,+4,+5).

Q11. How to look up a letter's position fast?
A11. Use five-letter blocks (A-E, F-J, K-O, …).

Q12. "Immediately followed by" means?
A12. The very next element in the string, with no gap.

Letter series is number series in disguise: convert letters to their positions (A=1 … Z=26) and the same difference-hunting works. The one skill that makes you fast is knowing the alphabet's positions cold.

What this topic tests: finding patterns in letter sequences, alphanumeric mixes, and continuous letter-gap series.

The method

Beginner — letter-to-number map

Assign A=1, B=2, …, Z=26. Then a series like C, F, I, L is 3, 6, 9, 12 (+3). Convert, find the numeric rule, convert back. Learn the anchor points: E=5, J=10, O=15, T=20, Y=25 — count from the nearest anchor.

Intermediate — reverse and paired patterns

• Reverse counting (Z=1, Y=2, …) or the mirror pair (EJOTY forward, and A↔Z, B↔Y opposites summing to 27).
• Two-letter groups: AZ, BY, CX → first letter +1, second letter −1 (opposite ends closing in).
• Skip patterns: A, C, F, J (gaps 2, 3, 4).

Advanced — alphanumeric and mixed series

Alphanumeric strings mix letters, digits and symbols; typical questions ask "which element is 3rd to the left of the 5th vowel" etc. Technique:

1. Write positions above the string.
2. Filter to the required type (vowels/digits/symbols) first.
3. Then count the offset. Doing the filter before the counting avoids miscounts.

Worked example

Find the next term: B, D, G, K, P, ?
Positions: 2, 4, 7, 11, 16 → differences 2, 3, 4, 5 (increasing by 1). Next difference = 6 ⇒ 16 + 6 = 22 ⇒ letter V.

Where it appears

TCS NQT, Infosys, Wipro, Capgemini reasoning; alphanumeric "count the symbol" items appear in most drives.

Speed tricks and shortcuts

• Memorise anchors E-J-O-T-Y (5-10-15-20-25) to place any letter in one step.
• Opposite letters sum to 27 (A+Z, B+Y, …) — instant mirror lookups.
• For alphanumeric counting, filter to the target type first, then count.
• Mnemonic: "EJOTY forward, and 27 minus position for the mirror."

Worked Example

Problem: Find the next term in the letter series: B, D, G, K, P, ?

Solution: Replace each letter with its position in the alphabet (A = 1, B = 2, …):

B = 2, D = 4, G = 7, K = 11, P = 16.

Now look at the gaps:

• 4 − 2 = 2
• 7 − 4 = 3
• 11 − 7 = 4
• 16 − 11 = 5

The gaps are 2, 3, 4, 5, so the next gap is 6.

Next position = 16 + 6 = 22, which corresponds to the letter V.

Answer: V.