It was the most popular ancient Greek wind instrument.
It consisted of two diverging pipes, suspended at their base. Each pipe had a separate mouthpiece and reed (single or double). In the unequal in length “auloi”, one played the melody while the other “supported” it. In the equal in length “auloi”, playing in the same register, a richer sound was achieved.
The “aulete” with great virtuosity, using circular breathing and blowing hard, achieved the harmonisation and the marvellous assonance of the two auloi. Often, he wore a kind of leather strap, the “phorbeia”, that went across his mouth (with two holes for the pipes), the cheeks and to the back of his head. Sometimes it was tied to an additional strap going over the top of his head for secure suspension.
The “diaulos” with its rich, intensely sentimental, rhythmical and penetrating sound was capable of accompanying choruses consisting of tens of men all on its own.

SOURCES: M.L. West, Ancient Greek Music, Curt Sachs, The History of Musical Instruments, Julius Pollux, Onomasticon, Athenaeus of Naucratis, Deipnosophistai, Aristotle, (Musical) Problems.