Well, I thought I wouldn't have to say anything, and that this question would be sufficiently answered, and yet I must add to Frankham's answer--

When you talk about the tritone not being used, that isn't exactly right--it hadn't been much used, melodically, relative to the fundamental (That is to say, in a piece in the key of C, the melody seldom jumped from C to F#), however, the possibility was always there when a Lydian mode was being used.

Harmonically, the Tritone has been used regularly since the introduction of the diatonic, major/minor scale system, since it is a key element in the Dominant harmony--I the key of C, the G7 chord is really built around the Tritone, B-F, and the G7 chord is really also a diminished B chord-- B-D-F, with a G added.

Don't believe me? Try this--play a simple C chord in the open position-- X-3-2-O-1-O which is C-E-G-C-e, then play X-2-3-X-3-X which is B-F-D--does that sound devilish?