Diatonic

A seven note musical scale based on (usually) the major mode of t-t-s-t-t-t-s intervals (t = tone, s = semitone).

Instruments tend to be diatonic or chromatic. A diatonic instrument is built to play the scale of a particular key e.g. G (see Melodeon or Penny Whistle).

A chromatic instrument can play all of the 12 semitones in an octave and therefore play any key or mode.