0A diatonic harmonica is typically a 10 hole harmonica which is tuned to play the major scale of its key. This means it is tuned to play only the natural notes and not the sharps and flats. However, the missing sharps and flats can be played using advanced note bending techniques. Generally, a diatonic harmonica uses a Richter tuning with some missing notes on lower and higher octave but there are solo tuned 12 hole diatonic harmonicas which are tuned to play all notes on lower, middle and higher octaves ! So technically, a tremolo harmonica is also a diatonic but without much of note bending ability :-)
