How do you write whole-notes in music?

A whole note is referred to as such because it is usually sustained for the entire measure. In 4 4 (also called common time), the note lasts for all 4 beats without stopping.

This is not the case when the piece is not in common time.

If it is in 2 4, the half note (open circle with a stem) fills the all 2 beats of the measure, but it is not called a whole note. The name remains.