That's pretty much the question. Thanks. --Otis
closed as off-topic by luser droog, Matthew Read♦ Jul 21 at 0:57
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Notes cannot be diatonic and chromatic simultaneously. This is like asking if a cat can also simultaneously be a dog. Since this does not occur (with the notable exception of Catdog,) the same may also be said for music theory.
Diatonic refers to pitches within a scale - such as a major / minor scale
Chromatic refers to all 12 chromatic pitches
If you were cheeky you could try and make the point that a chromatic scale is diatonic within itself, but it is a flimsy technicality at best and of course excludes the contingency of a tonal center. A chromatic scale might have a pitch center or pitch cell but it won't have a tonal center.
From here onward there could be endless discussion about which scales fall into which category; of which everyone would argue their own case.
In practical tradition, diatonic refers to and applies to the most common scales. Everything else is gray, and the chromatic scale does not apply.
A note is Diatonic when it is part of the key you are in. There are lots of ways to use the term chromatic... so
Usually within a key someone will say a note is chromatic when it doesn't come from the scale. Such as when using a secondary dominant chord.
When studying late-classical/romantic music you see the use of "chromaticism", which describes using notes/chords from outside your diatonic scale, often modal borrowing.
If you speak melodically, a diatonic note can be a part of a chromatic run or passage but is only chromatic melodically/linearly and at the time of the specific passage, not harmonically.
I'm sure we could find more ways that a note could be diatonic and chromatic at once but in reality they are often used to distinguish between each other. To say something is chromatic means that it is not diatonic.