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5.2.2 What Is a Diatonic Scale? It's a Heptatonic (7-Note) Scale with Diatonic Patterns
A diatonic scale is a heptatonic scale (7 different notes, not including the octave note at the upper end).
On the piano, suppose that, instead of playing
C – D – E – F – G – A – B – C,
you were to start on D and play only the white keys, like this:
D – E – F – G – A – B – C – D.
The pattern of tones and semitones shifts to:
Is this still the so-called “diatonic order”?
Yes it is. It's still a diatonic scale becase you still have five tones and two semitones (so it's heptatonic). They’re still spaced the same way.
But when you play this scale, it no longer sounds like the familiar “do-re mi” scale. It sounds a little weird, a little strange. By starting on a different note—D—you change the order of the frequency ratios of several of the notes of the scale with respect to the tonal centre.
It’s a different staircase.