Greybeard's pages

Any note can be flatted, so you'll see Cb from time to time.

Let's say you're playing in the key of F (F-G-A-Bb-C-D-E-F) and you want to play the note between Bb and C. You could call it B natural, since that's enharmonic to Cb, but there are times when it's easier to read if it's Cb.

The B's are already flatted by the key signature, so using the figure Bb-B-Bb in a measure means you'll need a natural, and then a flat, to get your idea across. Using Cb cuts the number of accidentals in half.

If you've got a repeated figure like Bb-B-Bb-B-Bb-B, it's difficult to read if there are lots of accidentals. By using two different letter names, you only write the accidental once or twice, depending on the key signature, and then you're good until the next bar line.

For much the same reasons, you'll sometimes see bb (double flat) or x (double sharp) in music.

The scale is broken up into intervals - firsts, seconds, thirds, etc. These are defined by the number of letters in the sequence, so a third has 3 letters in the sequence (take the C scale - C, D, E, F....., a third from C is E - C, D, E, 3 letters). In C, F is a 4th (C, D, E, F). The quality of the interval - major, minor, diminished or augmented - is ignored, here.
Take the scale of Ab - Ab, Bb, C, Db, Eb, F, G, Ab. The major chord triad is Ab, C, Eb. To make this an Abminor, I have to flatten the 3rd. This is C, so I would either have Cb or B. Using B, the chord would consist of a 1st (root), a sharp 2nd and 5th, which, by definition, is not an Ab minor. B would be a 2nd, because there are only two letters in the sequence A and B. The 2nd in Ab is a Bb and B is Bb, sharped. It would no longer be a 3rd (for that I need A, B, C) - even though the pitch of B and Cb are exactly the same.