well its the only standard chord (i.e. major, minor, augmented, diminished) that has a tritone, but other more complex chords can contain them. the Mm 7th chord for example has a tritone between the 3rd and the 7th
I agree with statzj37... The further you drift into chromatic harmony, the more frequently you'll encounter it. Don't forget the whole-tone (hexatonic) scale too, tritone intervals abound!
