In rockabilly, psychobilly, traditional blues and bluegrass music, the bassline is played by a double bass player. The bassline uses low notes that provide a rhythm while simultaneously setting out the foundation of the chord progression. The bassline bridges the gap between the rhythmic part played by the drummer and the melodic lines played by the lead guitarist and the chordal parts played by the rhythm guitarist and/or keyboard player. In most traditional and popular music styles, the bass player is expected to be able to improvise a bassline which they base in the chord progression of a song. When a bassist is playing a cover song, they may play the bassline that was originally used on the recording. A session bassist playing in a music studio is expected to be able to read a bassline written in musical notation. Bass players also perform fills in between the phrases of the vocal melody, and they may also perform bass runs or bass breaks, which are short solo sections. Rhythmic variations by the bass, such as the introduction of a syncopated figure can dramatically change the feel of a song, even for a simple groove. "In any style, the bass's role in the groove is the same: to keep time and to outline the tonality. When developing bass lines, these two things should always be your goal"  A walking bass is a style of bass accompaniment or line, common in Baroque music (1600–1750) and 20th century jazz, blues and rockabilly, which creates a feeling of regular quarter note movement, akin to the regular alternation of feet while walking. Thus walking basslines generally consist of unsyncopated notes of equal value, usually quarter notes (known in jazz as a "four feel"). Walking basslines use a mixture of scale tones, arpeggios, chromatic runs, and passing tones to outline the chord progression of a song or tune, often with a melodic shape that alternately rises and falls in pitch over several bars. To add variety to a walking bassline, bassists periodically interpolate various fills, such as playing scale or arpeggio fragments in swung eighth notes, plucking muted percussive grace notes (either one grace note or a "raked" sequence of two or three grace notes), or holding notes for two, three, or four beats. Some songs lend themselves to another type of variation: the pedal point, in which the bassist holds or repeats a single note (often the tonic or the dominant) under the chord changes.