When @jacksprat comments:
He has it right.
As an intro, before any backing track begins, it has no context and those three notes could be something different to an A7 chord. There is a certain ambiguity about it.
BUT
The sound of that 7th is a very recognisable one. And as soon as it is followed by the semitone movements down, it is very evidently a well-used blues cliche and fixes it as an A7 chord fragment. The A7 is implied.
Check the intro for Walking Easy Blues … it performs the same trick, the descending chromatic lead-in riff but this time beginning with a fragment that implies an E7 chord.
Where are these ascending parts? Everything after the intro is single notes.
Technically, yes. But it is just commonly referred to as a chromatic movement rather than naming each individually implied chord.
Follow along by reading the TAB as Justin plays and counts all parts from 09:50 in the video.
There is a time factor that you want to get the feel of when playing to the backing track. Without a backing track you have liberty to play around with the timing but don’t want to go too far off piste or it will sound sloppy.
I hope that helps.
Richard
