Open this file: les-hommes
It is the melody from a piece of Mozart. You’ll see that bars 1-4 (up to the double bar line) is section A. After the double bar line starts a new section for four bars. After that in bar 9 section A is repeated but an octave higher.
With the repeat marks at bar 4 you might write the structure like this:
AABABA – (without the repeat marks it would like this: ABA). This makes the structure Ternary.