Modulation (music)
Modulation, in music, means that the music changes key. A piece of music might, for example, be "in the key of C major" (meaning that it uses the notes of a C major scale, and the C sounds like the "home key" or "tonic" as it is called in music theory). Then it could modulate to G major so that the G now feels like the home key and the notes of a G major scale are used (the Fs will be F sharps).
Modulations like the one above are very common, because G is closely related to C (it is the 5th note in a C major scale: the "dominant"). A modulation to the subdominant (4th note of the scale) is also common (e.g. from C major to Fmajor). Music often modulates to the relative minor (e.g. C major to A minor).
A modulation that goes to a key whose tonic is not part of the original key is called a "chromatic modulation". Modulating from C major to A flat major would be a chromatic modulation because A flat is not a note in the C major scale.
Most pieces of music will modulate, especially if they are long pieces. It gives the music variety and helps to give it shape: the farther away from the tonic it goes the more tension there is. When the music eventually returns to the original key it feels like a homecoming.
Modulation (music) Media
Common-chord modulation in the opening of Chopin's Prelude in C minor, Op. 28, No. 20. Play (info • help)
Common-chord modulation in Tchaikovsky's Album pour enfants (1887), Op. 39, No. 10, Mazurka Play (info • help)
Common-chord modulation in the opening of Mozart's Sonata in D Major, K. 284, III Play (info • help)
Modulation from D major to D♭ major in Schubert's Op. 9, No. 14, D. 365, mm. 17–24, using the German sixth, in the new key, that is enharmonic to the dominant seventh in the old key. Play (info • help)
Common-tone modulation between chromatic mediants in Mozart's K.475 Play (info • help)