Double whole note

Left: breve in modern notation. Centre: breve in mensural notation used in some modern scores as well. Right: less common stylistic variant of the first form.

A double whole note (also called a breve) is a note that is double the value of a whole note, which is where it gets its name. In the $\frac{4}{4}$ time signature it has the value of 8 beats or two measures.