Chord progressions for bebop compositions were often taken directly from popular swing-era compositions and reused with a new and more complex melody, forming new compositions (see contrafact). This practice was already well-established in earlier jazz, but came to be central to the bebop style. The style made use of several relatively common chord progressions, such as blues (at base, I-IV-V, but 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 compositions. Bebop chord voicings often dispensed with the root and fifth tones, instead basing them on the leading intervals that defined the tonality of the chord. That opened up creative possibilities for harmonic improvisation such as tritone substitutions and use of diminished scale based improvised lines that could resolve to the key center in numerous and surprising ways.