Double whole note or breve

In medieval mensural notation, the brevis (ancestor of the modern breve) was one of the shortest note lengths (hence its name, which is the Latin etymon of "brief"). In "perfect" rhythmic mode the brevis was a third of a longa, or in "imperfect" mode half a longa (for full details of the complications here, see for example Hoppin 1978).[vague]. However, in modern music notation it is the longest note value still in common use.

In modern notation, a breve is represented in two ways: by a hollow oval note head, like a whole note, with one or two vertical lines on either side, as on the left of the image, and as the rectangular shape also found in older notation, shown in the middle of the image. An alternative notation consists of a joined pair of whole notes (at the right of the image), but this can easily be confused with a pair of unison whole notes, as found in double-stop notation for stringed instruments or when two parts are written on a single staff.

Because it lasts longer than a bar in most modern time signatures, the breve is now rarely encountered except in English music, where the half-note is often used as the beat unit. However, in time signatures where the top number is exactly twice that of the bottom, such as 4/2 or 8/4, it lasts a whole bar and so may still be found.