Key (music)

Popular songs and classical music from the common practice period are usually in one key.

Methods that establish the key for a particular piece can be complicated to explain and vary over music history.

For example, modern trumpets are usually in the key of B♭, since the notes produced without using the valves correspond to the harmonic series whose fundamental pitch is B♭.

(Such instruments are called transposing when their written notes differ from concert pitch.)

In the Baroque it was common to repeat an entire phrase of music, called a ritornello, in each key once it was established.

In Classical sonata form, the second key was typically marked with a contrasting theme.

In rock and popular music some pieces change back and forth, or modulate, between two keys.

Examples of this include Fleetwood Mac's "Dreams" and The Rolling Stones' "Under My Thumb".

"This phenomenon occurs when a feature that allows multiple interpretations of key (usually a diatonic set as pitch source) is accompanied by other, more precise evidence in support of each possible interpretation (such as the use of one note as the root of the initiating harmony and persistent use of another note as pitch of melodic resolution and root of the final harmony of each phrase).

Likewise, the horn, normally in the key of F, sounds notes a perfect fifth lower than written.

For example, a brass instrument built in B♭ plays a fundamental note of B♭, and can play notes in the harmonic series starting on B♭ without using valves, fingerholes, or slides to alter the length of the vibrating column of air.

Such "key coloration" was an essential part of much eighteenth- and nineteenth-century music and was described in treatises of the period.