Closely related keys can be found by looking at the circle of fourths. The three adjacent keys and their relative minors are the keys with the most in common, and modulated to most often because of this.

Common Chords Among Closely Related Keys

The following charts show the relationships of common chords among closely related keys. These chords would be used during a modulation using common tones (pivot chords). The first example uses C major as the relative tonic. The circled chord symbols are in the given major scale. The chord symbols outside of circles form the 13 chord minor when combined with the relative major chords:

This can be expanded with Roman Numeral Analysis to show common relationships in every key.