Rule of Odds
The observation that an odd number of subjects in a frame usually reads as more settled than an even number.
Measured in
count
In depth
With an even number the eye tends to pair elements and then compare the pairs, which creates a division; with an odd number one element sits between the others and the group reads as a whole. The effect is strongest at small numbers — three flowers versus four — and disappears entirely in a crowd.
Examples in practice
Three birds on a wire compose more comfortably than four, which the eye splits into two competing pairs.