Chord substitution involves playing a different set of chords for a given chord progression. This is useful for both beginners and advanced players. Beginners will be able to substitute easier chords for a given chord line (e.g., working from sheet music); and advanced players will be able to embellish a chord progression, often creating a "jazz" flavor.
The following table summarizes common substitutions for beginners. These substitutions can be used because the chords containing similar notes and hence have a similar sound.
Here is an example of simplifying a chord progression, by using the substitutions outlined in the previous table.
More advanced players can use a reverse of the table shown. E.g., for Am, play Am7, Am6 or perhaps use a combination of the two. This type of substitution is called direct substitution, and here are two examples: