Boogie-woogie is a style of jazz piano characterized by a percussive, rhythmic pattern in the bass (known as basso ostinato) and a series of improvised variations in the treble. It began in the mid- to late 1920s and flourished during the 1930s. Sometimes referred to as "the left hand of God" because of the left hand playing repeated bass patterns, while the right hand plays short melodic figures (riffs), the style was based on the blues progression, but was freely improvised. "Boogie" is derived from "bogey", meaning spirit; "woogie" was the name of pieces of wood tying railway tracks together. Many of the black piano players associated with the style traveled on the railroad from town to town, playing for "rent" parties (held by people to raise the rent money). Boogie-woogie strongly influenced the development of rockabilly and early rock'n'roll in the 1950s.