What scale/mode to play over a B major chord in the key of C

[Uncle Clownmeat]

I've been studying a music theory book about secondary dominants. In the key of C, the fifth of the third chord is a B major chord. It suggests using the harmonic minor scale that's a 4th above the root of the secondary dominant.

So, in the key of C, use the E harmonic minor scale when playing over a B major chord.

What do you all have to say about this, aside from "Play whatever sounds good."

[DupleMeter]

That's one way to look at it. The other is to play a B Mixolydian scale (B C# D# E F# G# A B).

In general, all secondary dominants are impersonating a V7 chord (even if the 7 is "inferred" and not actually in being played). All V7 chords take a Mixolydian scale.

[JonPR]

There's two ways to look at it, IMO.

1. Take the B major chord tones, and add four other notes from C major. B D# F#, plus C E G A. Yep, that's E harmonic minor.

2. What chord is following the B? I.e., what is its function? Normally in C major, it will be a secondary dominant, resolving to Em. So that also suggests E harmonic minor as the obvious choice. IOW, if Em is the next chord, no need to apply some fancy theory about "the harmonic minor scale that's a 4th above the root of the secondary dominant" - the choice is staring you in the face. E minor scale! with a D# to fit the B chord!

In jazz, of course, they might well choose some fancy altered scale for a B7 going to Em. C melodic minor is the same notes as B altered, and is of course interesting in a C major context because only one note needs altering: E down to Eb (D#) - as opposed to E harmonic minor, which is altering two notes from C major. Of course, you then miss the F# in the chord (because a "B7alt" chord contains F or G, not F#), so it depends on how important you think that F# is.
A jazzy alternative which keeps the F# is the B HW dim scale: B C D D# F F# G# A. Notice the G#, a strange choice in the context of both C major and E minor, but it works as 13th of B7, or a chromatic approach to the A.

The idea, of course, is to construct a melodic phrase to lead to the next chord, so an entire scale might not be necessary. I tend not to think in scales myself: just chord tones and passing notes.

[Uncle Clownmeat]

Thank you, John PR. Your post, especially points 1 and 2, are extremely helpful.