A Dictionary of Music and Musicians/A Due
A DUE (Ital., 'In two parts'), or A 2. This expression is used in two exactly opposite ways in orchestral scores. For the wind instruments, for which two parts are usually written on the same stave, it indicates that the two play in unison; for the strings, on the other hand, it shows that the whole mass, which usually plays in unison, is to be divided into two equal parts, the one taking the upper and the other the lower notes. In practice there is never any difficulty in seeing which meaning is intended.
[ E. P. ]