The act of sharping or flatting a ninth on a dominant seventh chord is often referred to as altering a chord. It is also typical to have an augmented fifth (more properly referred to as a flatted thirteenth). When a chord has these elements, it is in the classic form of an altered chord. Thus, when you're in a minor key, the V chord should normally be altered since this is more common and generally preferred with V chords in minor settings.

When a chord is altered, an interesting equality is produced between the chord tones of the altered chord and the chord tones of its potential tritone substitution. The only tones not identical here are the roots and fifths of each possible chord. However, when the sharp nine is utilized their specific roles become inverted. This only occurs when the altered chord is compared with its unaltered tritone substitution.

For example, a Gb7 altered (or Alt.) contains B 7.

All in all, what you should take away from this is that these chords are essentially interchangeable, and it is merely the root note, as a matter of emphasis, that makes the two options — one a given and one a substitute — two versions of the same underlying reality.

This situation affects your bass lines in two ways. First, it gives you the option of playing the actual dominant chord or the tritone substitute at will, at least in jazz music anyway. In the simplest language possible, this means that, when inserting dominant chords, you can play a proper fifth above the next destination chord or a half step (always one fret) above that same next chord. Second, this freedom allows you to walk through a series of ii-V or ii-V-i(I) progressions in a descending manner, which produces a smooth chromatic transition between each chord. Figure 18-5 shows you how this can be done tastefully.

Figure 18-5.

A succession of ii-V and ii-V-i(I) progressions using descending bass lines

TTS = Tritone Substitution