Quantitative Aptitude · Number Series, Coding-Decoding and Odd Man Out

Alphabet and Letter Series for CA Foundation

Updated 1 October 2026 · Fact-checked

Alphabet and letter series are patterns of letters where each term follows a rule on alphabet positions, such as a fixed skip, a forward or backward shift, or two interleaved patterns. Convert letters to numbers (A = 1 to Z = 26), find the gaps, spot the rule, and apply it to get the missing term.

Understand Alphabet and Letter Series

A letter series is a list of letters or letter groups arranged by a rule. Your job is to find the rule and give the next or missing term. The alphabet has 26 letters in a fixed order, so every letter has a position: A = 1, B = 2, and so on up to Z = 26.

Most series are just number series in disguise. Turn each letter into its position, look at the differences, and the rule usually shows up. For example, B, E, H, K has positions 2, 5, 8, 11. The gap is +3 every time.

The rule can take a few common forms:
- A constant skip forward or backward (such as +2 or -3).
- A gap that grows or shrinks (such as +1, +2, +3, ...).
- Two series mixed together, where odd-numbered terms follow one rule and even-numbered terms follow another.
- Letter groups, where each letter in the group moves by its own rule, for example the first letter moves forward while the second moves backward.

The alphabet is treated as a loop in many questions. After Z you continue from A, and before A you continue from Z. Keep this in mind when a series runs past either end.

Since this is an objective paper with 0.25 negative marking, speed and accuracy both matter. Learn the key positions by heart so you spend seconds, not minutes, on each question.

Key formulas to remember

Alphabet position: A = 1, B = 2, ..., Z = 26; Memory aid: EJOTY gives E = 5, J = 10, O = 15, T = 20, Y = 25. Count from the nearest anchor.
Opposite letter (mirror): Position of opposite letter = 27 − position of the letter; A ↔ Z, B ↔ Y, M ↔ N. The two positions always add up to 27.
Forward or backward shift: New position = old position ± n; Use + for moving toward Z and − for moving toward A.
Wrap-around rule: If the result is more than 26, subtract 26. If it is less than 1, add 26.; Example: Y (25) + 3 = 28, and 28 − 26 = 2, so the letter is B.
Gap between terms: Gap = position of next term − position of current term; Check whether the gap is constant, growing, or alternating before deciding on the rule.
Alternating series: Odd-numbered terms form one series, even-numbered terms form another; Use this when a single gap does not fit all terms.

How to solve Alphabet and Letter Series questions

Use this method for any letter series. It works for single letters, letter groups and mixed patterns.

1Write the position number under each letter. Use EJOTY anchors to do this quickly.
2Find the gap between each pair of consecutive terms. Write the gaps down.
3If the gaps are the same, the rule is a constant skip. If they change by a fixed amount, the rule is a growing or shrinking gap.
4If the gaps look random, split the series into odd-numbered and even-numbered terms and check each part separately.
5For letter groups, treat each position in the group as its own series (first letters, second letters, and so on).
6Apply the rule to find the missing term. Use the wrap-around rule if the position goes above 26 or below 1.
7Check that the rule fits every given term, not just the first few. Then match your answer to the options.

Quickest way: Position-gap scan with option elimination

When to use it: Use this in the MCQ paper for almost every letter series. It saves time and cuts down guesswork.

Look at the first three terms and mentally note the gaps. Most series show their rule within two gaps.
If the gaps do not match, jump straight to splitting into alternate terms.
For letter groups, check the first letters only. This often narrows the options to one or two.
Check the options for the position of the missing term. If only one option has the right first letter, pick it.
Skip any question where the rule is not clear after about 40 seconds. A wrong answer costs 0.25 marks, so come back if time remains.

Common mistakes in Alphabet and Letter Series

Counting from A every time and miscounting positions.
Students do not memorise anchor letters, so they count on fingers and slip by one.
Fix: Learn EJOTY (E 5, J 10, O 15, T 20, Y 25) and count from the nearest anchor.
Forgetting to wrap around after Z (or before A).
Students treat the alphabet as a straight line that ends at Z.
Fix: If the position goes above 26, subtract 26. If it goes below 1, add 26.
Assuming a single constant gap when the series is alternating.
The first two gaps look equal, so students stop checking.
Fix: Test the rule against every term. If it fails, split into odd and even terms.
Mixing up the direction of the shift in letter groups.
One letter moves forward while the other moves backward, and students apply the same sign to both.
Fix: Work out the gap for each letter position of the group separately and note its sign.
Using the wrong term number when the series is interleaved.
Students pick the wrong sub-series for the missing term.
Fix: Count which term number is missing. Odd numbers belong to the first series, even numbers to the second.
Using the mirror rule as 26 − position.
Students forget that A is 1, not 0.
Fix: Use 27 − position. Check: A (1) and Z (26) add up to 27.

Worked examples

Example 1

Find the next term: B, E, H, K, ? Options: (a) M (b) N (c) O (d) P

Show the solution

Positions: B = 2, E = 5, H = 8, K = 11.
Gaps: 5 − 2 = 3, 8 − 5 = 3, 11 − 8 = 3. The gap is a constant +3.
Next position = 11 + 3 = 14.
Position 14 is N (M = 13, N = 14).

Answer: (b) N

Example 2

Find the next term: AZ, BY, CX, DW, ? Options: (a) EV (b) EU (c) FV (d) DV

Show the solution

First letters: A, B, C, D have positions 1, 2, 3, 4. Each moves +1, so the next is 5 = E.
Second letters: Z, Y, X, W have positions 26, 25, 24, 23. Each moves −1, so the next is 22.
Position 22 is V (U = 21, V = 22).
Check with the mirror rule: 5 + 22 = 27, which matches the pattern in every earlier pair.

Answer: (a) EV

Example 3

Find the next term: C, Z, F, W, I, T, ? Options: (a) L (b) Q (c) M (d) R

Show the solution

The gaps between consecutive terms are not constant, so split into odd-numbered and even-numbered terms.
Odd-numbered terms: C, F, I have positions 3, 6, 9. The gap is +3.
Even-numbered terms: Z, W, T have positions 26, 23, 20. The gap is −3.
The given series has 6 terms, so the missing term is the 7th term, which is an odd-numbered term.
Next odd-numbered position = 9 + 3 = 12, which is L.

Answer: (a) L

Exam tips

Memorise the positions of A to Z, and the EJOTY anchors, before you practise. This alone saves most of your time.
Check the options early. If three options have a different first letter from the likely answer, you can often finish the question after finding one letter.
When the gaps do not fit, try splitting the series into alternate terms before trying anything complex.
Watch for wrap-around in long series. If the answer looks odd, check whether the position crossed 26.
Do not spend more than about a minute on a stubborn question. Skip it and return later, since wrong answers carry 0.25 negative marking.

Practice questions from Number Series, Coding-Decoding and Odd Man Out

Alphabet and Letter Series: frequently asked questions

How do I solve letter series questions quickly?

Convert letters to alphabet positions and find the gaps between terms. If the gaps are not constant, split the series into odd and even terms. Memorising the EJOTY anchors makes the conversion very fast.

What is the position value of each letter?

A is 1, B is 2, and so on up to Z, which is 26. A useful trick is EJOTY, where E = 5, J = 10, O = 15, T = 20 and Y = 25.

How do I find the opposite letter in the alphabet?

Subtract the position of the letter from 27. For example, the opposite of D (4) is the letter at position 23, which is W. The two positions always add up to 27.

Is the alphabet treated as a loop in the exam?

Often yes, for series that run past Z. In that case, you continue from A, so subtract 26 from any position above 26. Check that your answer fits the pattern of the other terms.