Audio: diatonic (0:06)
diatonic plays a melody on a bandoneon or tango accordion. The diatonic figure shows that there are eight different notes in the score. Seven of the notes, those in the first and second bars, are diatonic to the key of C major. The eighth note, D# in the third bar, is chromatic in the key of C.
Diatonic is a weasel word. It is used in quite different ways.
A diatonic scale is a term used to collectively describe the seven modes. The seven modes all contain the same sequence of whole tones and semitones and each mode starts on a different note. The term, diatonic scale, is used to describe the underlying sequence of whole tones and semitones. All seven modes are diatonic scales and a diatonic scale has seven notes. Used in this sense, the terms, diatonic scale and mode, mean the same thing. The guide does not use the term, diatonic scale, it uses the term, mode, because it is shorter and simpler.
There is some debate whether minor scales are diatonic scales. Certainly, the natural minor scale is a diatonic scale because it is also a mode, the aeolian mode. The issue is whether the harmonic and melodic minor scales are diatonic scales. Some writers treat them as diatonic scales along with the rest of the modes, others treat them as nondiatonic scales because they are artificial scales and not modes. This is another reason for not using the term, diatonic scale, in the guide, it is ambiguous.
There is a useful distinction to be made between diatonic and chromatic. Diatonic is used to describe a note that is part of the scale or key under consideration, chromatic describes a note that is not. It means that seven notes are diatonic to a key, and five notes are chromatic and not part of the key. This is the sense that diatonic and chromatic are used in the guide. Diatonic means a note is part of the prevailing key, chromatic means a note is not. This seems simple enough.