What is the unit of repeat?
The unit of repeat is the smallest block that can be repeated to make the whole pattern. In circle, square, circle, square… it is circle–square. Four shapes would work too, but they would not be the smallest block.
Think of a striped scarf: you want the little sequence the knitter keeps using, not a description of the entire scarf.
Find the smallest repeating block
Look at AB | AB | AB. The little block AB repeats. It contains two letters, so the pattern repeats every two places. That length is called its period. The shortest working length is the minimum period.
Now group AAB | AAB | AAB. Repeating just A would miss the B. Repeating AA would miss it too. The smallest block that works is AAB, with three places. Its minimum period is 3.
Try a small block first. Repeat it and check each letter against the example. If a letter does not match, try a different block. The NCETM guide to patterns explains why finding the smallest unit matters more than naming every item.
Two circles in a row are not always a mistake
Try the unit circle–square–circle. Repeat it and you get circle–square–circle | circle–square–circle. Two circles meet at the join. Looking only at that pair could send you down the wrong path.
A short sequence can also fit several rules. These examples ask you to repeat a block. In a different puzzle, the pattern might follow another rule. Read the instructions first.
What comes after ABBABBAB?
B, if the rule is to repeat ABB. Group it as ABB | ABB | AB: the final group is missing its last B. The visible sequence does not need to end at a complete block.
A pattern that grows, such as 2, 4, 6, 8, needs a different explanation. Next, try checking every step in a number pattern, or put your eye for patterns to work in the Reasoning Challenge.