Double whole note
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In music, a double whole note (American or "German" terminology) or breve (British or "classical" terminology) is a note lasting twice as long as a whole note (or semibreve). 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 cognate of "brief"), and could be either a half or a third as long as the longa. However, in modern music notation it is the longest note with a distinct symbol.
In modern notation, a breve is represented by a hollow oval note head, like a whole note, with one or two vertical lines either side (as on the left of the image). In older notation, it is represented by an oblong shape (as shown in the middle of the image). An alternative notation consists of two adjacent hollow oval note heads (the right of the image).
Because it lasts longer than a bar in most modern time signatures, the breve is now rarely encountered. In time signatures where the top number is exactly twice that of the bottom, however, it lasts a whole bar and so may still be found (the most common time signature of this kind is 4/2).
A related symbol is the double whole rest (or breve rest), which usually denotes a silence for the same duration. Double whole rests are drawn as filled-in rectangles occupying the whole space between the second and third lines from the top of the musical staff. They are often used in long silent passages which are not divided into bars to indicate a rest twice as long as a whole rest (or semibreve rest). This and longer rests are collectively known as measure rests.
Alla breve, used to refer to the time signature 2/2, comes from the note value breve. Originally, in mensural notation, it meant that the brevis was to be considered the unit of time (tactus), instead of the semibrevis, as in other time signatures. This corresponds to its modern use, except that the breves have been replaced by minims (half notes) as the longer note values were discontinued. The sign is similarly a vestige of the one used in mensural notation.