I used the tri-tone substitution for years before I knew the name of it, and perhaps you do also. It makes sense, because it allows you to “slide in” to the target chord for a smoother transition. In fact, I always refered to it as a half-step slide from above when moving from a V7 chord to a I chord.

A tri-tone substitution is the use in a chord progression of a 7th chord that is three whole steps (in other words, a tri-tone) away from the original 7th chord. For example, if you were moving from G7 to C, you would substitute a Db7 for the G7 chord before you land on the target chord C.

The reason these chords may be substituted for each other is that they have two notes in common — and the two notes are always a tri-tone (3 whole steps) away from each other. In a G7 chord, the third is B and the seventh is F. In the Db7 chord, the third is an F and the seventh is Cb (same sound as B, but written enharmonically).
Watch the short video, and I think you will understand.

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