In phonetics, a diphone is an adjacent pair of phones. It is usually used to refer a recording of the transition between two phones.

In the following diagram, a stream of phones are represented by P1, P2, etc., and the corresponding diphones are represented by D1-2, D2-3, etc:

 |P1===|P2===|P3===|P4===|P5===|P6===| ===|D1-2=|D2-3=|D3-4=|D4-5=|D5-6=|=== 

If the number of phones in a language is P, then the theoretical number of possible diphones is P², although since all languages have restrictions about what sounds can occur next to each other, the number of diphones in each language is usually much smaller than P².

Spanish has about 800 diphones and German has about 2,500.

Diphones are useful in speech synthesis because combining pre-recorded diphones to create synthesized speech sounds much more natural than combining just simple phones, because the pronunciations of each phones varies based on the surrounding phones.