Question Type 9: A Letter Series

Eleven-Plus Preparation Specialists

This type of question relies solely on your logic and reasoning, not on your vocabulary. It’s therefore very well suited to those that are naturally good at logic, or those more inclined to Maths than English.

What does this question type involve?

In this type of question you will be asked to complete a series of letters in the way that makes the most sense. You will be given a series of pairs of letters. You must then add another pair. The series can be developed in a variety of different ways, and the key is understanding that.

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Example questions

Example: BB CA DZ EY
Answer: FW

Here the pattern is +1 for the first letter and -1 for the second letter.

Example: AA, BC, CE, DG
Answer: EH

Here the pattern is +1 for the first letter and +2 for the second letter.

3 Step Approach to Letter Series Questions

These questions can be confusing, and without a clear idea of the common patterns you will struggle to pick up all the marks. However, if you know the common patterns and can work through them systematically, then you’ll find this section much more straightforward. We recommend that you take a three step approach to this type of question:

Step 1: Forward or Backward
Step 2: Distance
Step 3: Variations

Your first step is checking whether the letters are moving forward (+) or backwards (-). Remember that the letters can move through from Z to A or back from A to Z. Look at both letters and make a decision. Remember that the letters can move independently. Now, you need to move on to the second step.

You need to calculate the distance that each letter is moving backwards or forwards each time. There has to be a logical pattern here, so calmly work through to find it out. If needed, you can note down how far it moves to help. Commonly, you will find +1, +2, +3 etc, +2, + 4, + 6 etc, +1,+3, +5, etc, or at the most complex +1,+2,+3, +5, +8 (movement amount is the two before added together). Remember that the same process can be followed for moving backwards as well. Calculating the distance should allow you to now find the answer – you’ll have a pattern for both numbers.

However, if you still haven’t solved the puzzle, then there is one remaining possibility. The numbers might be moving with relation to a different point in the alphabet. A mirror (i.e. reflecting letters across a certain point in the alphabet) is unlikely to be used in this question type, but be aware of them nonetheless.

Worked Example

Find the next pair in the sequence:

So, first of all we need to see if the letters are moving backwards or forwards. It should be obvious that the letters are moving in different directions, with C moving forward to become an E and the E moving backwards to become an A. However, don’t be confused by the next step, which sees the A move backwards again to become a W.

So, now we need to see how far the movement is. C to E to G is +2 each time. Looking at the second letter, E to A to W is minus 4 each time.

Therefore, we don’t need to look for anything more complex, like a sequence that changes (e.g. +1,+2,+3) or the letters being centred around a particular point.

We can see that the answer must be IS. 

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Top Tricks

Always try simple things first. Often these questions are very simple, yet students are primed and ready for the most complex iterations. Instead, focus on +1, +1 etc first – and don’t doubt yourself if you think you’ve spotted the pattern immediately.

Common Pitfalls

Remember to be aware of the letters moving backwards through A→Z or forwards from Z→A. This can cause confusion and make students think that the pattern has changed dramatically, when in reality it has remained the same.


In summary, work logically and be aware of the common patterns. Work through the three step process. You should practise beforehand to ensure that you become used to spotting the most common patterns.
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