The dominant key in a given musical composition is the key

Key (music)

In music theory, the term key is used in many different and sometimes contradictory ways. A common use is to speak of music as being "in" a specific key, such as in the key of C major or in the key of F-sharp. Sometimes the terms "major" or "minor" are appended, as in the key of A minor or in the...

 whose tonic is a perfect fifth above (or a perfect fourth below) the tonic of the main key of the piece. Put another way, the key whose tonic is the dominant

Dominant (music)

In music, the dominant is the fifth scale degree of the diatonic scale, called "dominant" because it is next in importance to the tonic,and a dominant chord is any chord built upon that pitch, using the notes of the same diatonic scale...

 scale degree in the main key.

If, for example, a piece is written in the key of C major, then the key of C is the tonic key. The key of G major is the dominant key since it is based on the dominant note for the key of C major.

In sonata form

Sonata form

Sonata form is a large-scale musical structure used widely since the middle of the 18th century . While it is typically used in the first movement of multi-movement pieces, it is sometimes used in subsequent movements as well—particularly the final movement...

 in major keys, the second subject group is usually in the dominant key. Even with the widest roaming modulations

Modulation (music)

In music, modulation is most commonly the act or process of changing from one key to another. This may or may not be accompanied by a change in key signature. Modulations articulate or create the structure or form of many pieces, as well as add interest...

 in the development

Musical development

In European classical music, musical development is a process by which a musical idea is communicated in the course of a composition. It refers to the transformation and restatement of initial material, and is often contrasted with musical variation, which is a slightly different means to the same...

, the dominant key exerts influence and eventually forces a return to the tonic key.