About Letter and Symbol Series
A letter series is a number series written in the alphabet. The letters look unfamiliar, but once each one is replaced by its position, 1 to 26, the question is the same as any number series: find the rule that every visible term obeys, then apply it once more. The letters are what make candidates hesitate, so writing the numbers down first is the whole battle.
What you need to understand
- Replace every letter with its alphabet position before looking for a pattern. Almost every rule is about position.
- Equal gaps between the positions mean a constant step, exactly as in a number series.
- Gaps that themselves increase by a fixed amount mean the series accelerates; the rule is in the gaps.
- A series can run backwards, in which case the steps are negative and the positions fall.
- Some series carry a number beside each letter, such as A1, C3, E5. The letter and the number then follow two separate rules and must be solved independently.
- A change of case from one term to the next is a presentation pattern, not the rule itself.
- As always, test the simplest rule that fits every visible term rather than the first rule that fits the last two.
How to work through these questions
- Write the alphabet positions under each letter: A as 1, B as 2, and so on.
- Write the gaps between consecutive positions directly below them.
- If the gaps are equal, add that gap to the last position and convert the result back to a letter.
- If the gaps are not equal, look at how the gaps change from one to the next.
- If every term has a letter and a number, split the question in two and solve each sequence on its own.
- Convert your answer back to a letter and check it against every visible term before choosing an option.
Mistakes that cost marks
- Assuming a constant gap from the first two letters alone, when the later terms move further each time.
- Treating a paired term such as C3 as one value instead of two independent sequences.
- Miscounting the alphabet, most often around M and N where counting drifts by one.
- Missing an alternating pattern in which two different rules take turns.
- Forgetting that a letter series can count downwards, so the step is negative rather than positive.
Worked example
Find the next letter in the series: A, C, F, J, ?
- Write the alphabet positions under the letters: 1, 3, 6, 10.
- Write the gaps between consecutive positions: 2, 3, 4.
- The gaps are not equal, but they increase by one each time, so the next gap is 5.
- Add the gap to the last position: 10 + 5 = 15.
- The fifteenth letter of the alphabet is O, and checking the rule against every term confirms it.
Answer: O
Practice questions with answers
A few Letter and Symbol Series questions with the full solution shown, so you can see
how the method is applied before you attempt the timed set.
Question 1
Which letter comes next in the series: H, L, P, T, ?
Answer: Option B — with explanation
Each letter is 4 places ahead of the one before it, so the gaps are constant at 4.
T is letter 20 of the alphabet; 4 places on gives letter 24, which is X.
Common mistakes
- repeated the last letter shown: T
- stepped only 3 places instead of 4: W
- stepped 5 places instead of 4: Y
Question 2
Work out the next term of the series: B, E, I, N, ?
Answer: Option C — with explanation
The gaps between the letters grow by one each time: 3, 4, 5, 6.
N is letter 14; 6 places on gives letter 20, which is T.
Common mistakes
- widened the gap by two instead of one: U
- kept the gap the same instead of widening it: S
- repeated the last letter shown: N
Question 3
Find the next letter in the series: W, T, Q, N, ?
Answer: Option D — with explanation
The series counts backwards, each letter 3 places before the last.
N is letter 14; 3 places back gives letter 11, which is K.
Common mistakes
- repeated the last letter shown: N
- moved forward 3 places instead of backward: L
- moved back 4 places instead of 3: J
Question 4
Which letter comes next in the series: E5, F9, G13, H17, ?
Answer: Option A — with explanation
The letters and the numbers move independently. The letter advances 1 place(s) each time and the number rises by 4.
So after H17 comes I21.
Common mistakes
- left the letter unchanged: H21
- advanced the letter 2 places instead of 1: J21
- left the number unchanged: I17
Question 5
What should replace the question mark (?) in the series: B, G, L, Q, ?
Answer: Option A — with explanation
Each letter is 5 places ahead of the one before it, so the gaps are constant at 5.
Q is letter 17 of the alphabet; 5 places on gives letter 22, which is V.
Common mistakes
- stepped 6 places instead of 5: W
- stepped only 4 places instead of 5: U
- doubled the gap a second time: S
Frequently asked questions
How do I solve a letter series quickly?
Write the alphabet positions under the letters and then treat the question as an ordinary number series. The gap between consecutive positions is the rule in the great majority of questions, and writing the numbers down takes only a few seconds.
What if the series mixes letters and numbers?
Solve it as two separate series running side by side. Work out the rule for the letters and the rule for the numbers independently, then combine the two results for the answer.
Do I need to memorise the alphabet positions?
It helps, but it is not essential. Knowing the positions of the letters at intervals of five lets you reach any letter within a step or two, which is fast enough for an exam.
What does an alternating letter series look like?
Two rules take turns, so the gaps form a pattern such as 2, 5, 2, 5. Splitting the series into odd and even positions often makes the two rules obvious.
Can a letter series count backwards?
Yes, and it is easy to miss. If the positions fall rather than rise, the step is negative, and you continue subtracting. The series simply runs towards A instead of Z.
