Bebop musicians employed several harmonic devices which were not previously typical in jazz, engaging in a more abstracted form of chord-based improvisation. Bebop scales are traditional scales with an added chromatic passing note; bebop also uses "passing" chords, substitute chords, and altered chords. New forms of chromaticism and dissonance were introduced into jazz, and the dissonant tritone (or "flatted fifth") interval became the "most important interval of bebop" Chord progressions for bebop tunes were often taken directly from popular swing-era tunes and reused with a new and more complex melody and/or reharmonized with more complex chord progressions to form new compositions, a practice which was already well-established in earlier jazz, but came to be central to the bebop style. Bebop made use of several relatively common chord progressions, such as blues (at base, I-IV-V, but often infused with ii-V motion) and "rhythm changes" (I-VI-ii-V) – the chords to the 1930s pop standard "I Got Rhythm". Late bop also moved towards extended forms that represented a departure from pop and show tunes.