A common chord, in some later usage, is a chord that appears in more than one key.
Common chords, in this sense, are frequently used in modulations, in a type of modulation known as common chord modulation or diatonic pivot chord modulation. It moves from the original key to the destination key (usually a closely related key) by way of a chord both keys share. For example, G major and D major share 4 chords in common: GMaj, Bmin, DMaj, Emin. This can be easily determined by a chart similar to the one below, which compares chord qualities. The I chord in G Major—a G major chord—is also the IV chord in D major, so I in G major and IV in D major are aligned on the chart.
Any chord with the same root note and chord quality can be used as the "pivot chord." However, chords that are not generally found in the style of the piece (for example, major VII chords in a Bach-style chorale) are also not likely to be chosen as the pivot chord. The most common pivot chords are the predominant chords (ii and IV) in the new key. When analyzing a piece that uses this style of modulation, the common chord is labeled with its function in both the original and the destination keys, as it can be seen either way.
The number of diatonically occurring chords that two keys share is a measure of how closely related they are. A closely related key can be defined as one that has many common chords. A relative major or minor key has all of its chords in common; a dominant or subdominant key has four in common. Less closely related keys have two or fewer chords in common.