An altered bass chord (aka "slash chord") is written the same way as an inverted chord, like this: C/D
and looks like this:
The D note is not part of the C chord (C E G), i.e. it's a non-chord note, so C/D just means a C chord played simultaneously with a D note in the bass.
According to Michael Miller in his book The Complete Idiot's Guide to Music Composition, when choosing a "pleasing" non-chord bass note, "seconds and fourths of the chord typically sound better than sixths and sevenths".
This means that in the key of C, "pleasing sounding" bass notes would be D (second) and F (fourth), while A (sixth) and B (seventh) would be less pleasing. Just for clarification, here's the C scale:
C (root) D (second) E (third) F (fourth) G (fifth) A (sixth) B (seventh)
Here's an audio example of the 4 different "non-chord-bass-note slash chords" for the key of C: C/D (D3 4CEG), C/F (F3 4CEG), C/A (A3 4CEG), C/B (B3 4CEG).
Personally, I think they all sound pleasing, but see what you think.
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