A quick (but probably quite expansive) question here on bluesy jazz progressions. I've noticed that many times in a jazz blues progression chart the stardard 12 bar blues chords will be stated however the player often plays more than one chord on each stated chord... So for example in a blues in Ab it will go:
| Ab - - - | Db - - - | Ab - - - | Ab - - - |
But then in bar 5 when it is meant to be Db for 4 beats it may use two different chords over the Db but it's not stated on the chart:
| Db - (X chord) - |
It sounds great when players do this but leaves me feeling bewildered as to what chord they're actually playing to keep the progression moving. Does anyone know whats going on here? Would they use an inversion? Substitution maybe? Any help would be greatly appreciated.