Closely related key

Distant keys may be reached sequentially through closely related keys by chain modulation, for example, C to G to D.[2] For example, "One principle that every composer of Haydn's day [Classical music era] kept in mind was over-all unity of tonality.

In modern music, the closeness of a relation between any two keys or sets of pitches may be determined by the number of tones they share in common, which allows one to consider modulations not occurring in standard major-minor tonality.

However, modulating up a tritone would produce F♯, G♯, A♯, C♯, D♯, which shares no common tones with the original scale.

According to another view in modern music, notably in Bartók, a common tonic produces closely related keys, the other scales being the six other modes.

Andranik Tangian suggests 3D and 2D visualizations of key/chord proximity for both all major and all minor keys/chords by locating them along a single subdominant-dominant axis, which wraps a torus that is then unfolded.