Circles
Circles are the 'bread and butter' of school work.
Although they look as if they should be easy, executing circles with the correct bend, that are circular, keeping the horse forward going, and not falling in (veering towards the centre of the circle) is not as simple as it looks.

Circles can be any size but 10m, 15m and 20m are commom as they are easier to judge against the dimensions of the school.
Next - serpentines.