Tri-Tony

A large influence on Bernstein's music for "West Side Story" was the tritone.  Tritones are pitches that are separated by three (tri-) whole tones.  That interval between pitches is called an augmented fourth, as seen below.

Augmented 4th

An F# is three whole tones above a C (C-D-E-F#).  However, some tritones are separated by 6 semi-tones instead, like the diminished fifth seen below.

Diminished 5th

The Gb is six semi-tones above the C (C-Db-D-Eb-E-F-Gb).

Tritones are also part of very common chords in music like the dominant seventh chord shown below.

C7 Chord

The dominant seventh chord contains a tritone between the third and seventh of the chord (the interval is a diminished fifth).  However if this chord were in second or third inversion, the interval would change from a diminished fifth to an augmented fourth.