What term refers to the extensive use of notes outside the key signature, particularly in 19th-century music?

The term that refers to the extensive use of notes outside the key signature, particularly in 19th-century music, is chromaticism. This musical technique involves the use of pitches that are not part of the standard scale or key, which adds a sense of tension and complexity to the harmony. In 19th-century compositions, composers frequently employed chromaticism to create rich emotional landscapes and to evoke specific feelings, moving beyond traditional diatonic scales.

The use of chromatic notes allows for a variety of expressive possibilities; composers could incorporate unexpected harmonies and create smooth voice leading by utilizing half steps, which are characteristic of chromaticism. This contrasts with modulation, which involves changing from one key to another but does not inherently imply the same level of harmonic complexity or adventurousness outside the original key. Tonality is the overarching system governing key relationships in music, while modal interchange refers to the borrowing of chords from parallel modes, which does not specifically focus on the extensive use of non-diatonic notes in the same way that chromaticism does.