Modulation, in music, is the phenomenon of changing the tonality of the music—that is, changing what key it's in.
In Western classical music tradition (on which much of today's popular music worldwide is based), modulations are usually between keys that are close to each other—that is, they differ by at most one or two sharps or flats, if at all. These feel natural because the two keys have a lot of pitches in common with each other (for example, modulating from the key of C to the key of Gnote ). As well, closely related keys have chords in common, so it makes the transition sound smoother. For example, the keys of C major and G major both contain the chords of C major, G major, 'a minor' and 'e minor'.
One prominent exception to this is going between the major and minor keys of the same "home note" (e.g. C major and C minor) - but that's not really a modulation anyway, just a mode shift.
Going into the 19th century, musicians started to experiment with more remote modulations, such as going from a key with no flats or sharps and then modulating to a key with a number of sharps or flats (e.g., from C major to A♭ major). These remote modulations are harder to pull off, because there's fewer chords in common between the keys. One related trend that helped make remote modulations more feasible was the emancipation of dissonance. By the late 19th century, a much wider range of harmonies were deemed acceptable, including chords with dissonant intervals. Moreover, some of these dissonant chords were ambiguous and thereby could create a way to escape from the orbit of closely-related keys and explore more remote keys. For example, playing a fully diminished seventh chord in the key of C major, the 'b diminished seventh' chord, can resolve to several different chords.
This culminated in the 20th century where songs sometimes had little clear sense of a key, or even, with atonal music, an avant-garde style, didn't have a key at all. Arnold Schoenberg wrote in an atonal style called 12 tone music.
The resulting repertoire of common modulations grew quickly. In the 1920s and 1930s, swing jazz incorporated many of the chromatic harmonic innovations from the late-19th century music of somposers such as Liszt and Chopin, including diminished and half-diminished chords and dominant chords with added notes. By the 1960s and 1970s, a range of rock songwriters, jazz rock groups and progressive rock bands also explored this wider range of harmonies. This was also where the Truck Driver's Gear Change, the business of modulating up by a mere half-step, began to gain ground. These were unlike prior modulations because they didn't add a sharp to the key signature, they added a sharp to every note being played. Typically, this means shifting from a sharp key to a flat key or vice versa, which would have given some classical composers apoplexies (indeed, if you were to plot this type of modulation on the Circle of Fifths, you would be jumping to a position almost directly opposite the starting key), but it works because the keys are so similar.
Usually, you can tell a modulation if the "flavor" of the music (and usually, though not necessarily, the emotion) changes. This doesn't happen too often in modern popular songs, partly because they're rather short and partly because it's hard to do them smoothly, but can sometimes be found between verses and refrains, with verses in one key and refrains in another. Examples of this technique include The Beatles' "Good Day Sunshine", Boy Band Midnight Sons' "If Only Tears Could Bring You Back", and—most notably for tropers—Jonathan Coulton's "Still Alive" and "Want You Gone." (These two get extra credit for switching between sharp keys and flat keys: D to F in the first, B♭ to G in the second.)
Modulations are very common in classical music, being a staple of tonal music (i.e. dating from the late 1500s forward, as opposed to modal music, which cannot change keys by virtue of not having a key in the first place). They occur both in miniature (lasting only a couple bars, or even a couple beats) and larger-scale (for entire sections of a work) forms. However, since at least the Seventies, the key change has been going out of style and has been all but erased from the Pop charts. Data analyst Ari Shapiro listened to all the #1 hit songs of The New '10s for key changes, and found only one ("Sicko Mode"). Some styles have few or no modulations at all, such as funk and Three Chords and the Truth genres such as Punk Rock. Other popular music styles have lots of modulations, such as jazz, jazz fusion and progressive rock ("Prog").
Modulation tropes include:
