Chord substitution is when you swap one chord with another that is very similar to it. In practice, this is usually accomplished by adding a note or two, which helps give the chord just a little extra character that it would not have had otherwise.
In this lesson, we'll build on the dominant 7th shape that we learned earlier.
Here's a quick introduction to one way you can use I IV V, although there is a TON more to say on this topic. The guitar is tuned to 4ths, except for the G to B string, which is a major third. That means that every time you switch strings, staying on the same fret, you're moving up (or down) a fourth.
For example, on 6th string, 5th fret, you have an A. Move up one string to the 5th string, and you have the D. Now, move up two frets, and you find the E, and there's your I IV V in the key of A, the three most important chords in that key. This works in both major and minor keys.
Learning a bit about I IV V can really help you reduce how much you need to memorize your fretboard, because you'll already know the relationships of the notes nearby. Cool stuff!
In this lesson we'll learn the E7 chord, in a couple different shapes, and how to shift that chord into 12 new chords, by transposing the chord shape to other places on the fretboard. You can also call this chord E dominant 7th. The basic shape of the open E7 is 020100.