Law of Similarity
The gestalt principle that elements sharing shape, colour, size or texture are perceived as related.
In depth
Similarity lets a photographer create a visual link between elements that are physically separated — three red objects in different corners will be read as a set. It also means that the one element which does not match becomes conspicuous, which is the mechanism behind almost every 'odd one out' photograph.
Examples in practice
A row of identical yellow chairs with one blue chair among them makes the blue one the subject automatically.