Leitmotifs

A leitmotif (/ˌlaɪtmoʊˈtiːf/) is a musical "phrase" or melody, re-used in multiple songs to associate a tune with a person, event, or place. The use of leitmotifs in Undertale's soundtrack and other sound files is extensive, creating a nostalgic mood for the game and helping evoke emotional responses in players. Toby Fox has been noted stating that the re-use of melodies has made his composition of the soundtrack much easier,