This study examines the ways in which jazz musicians use rhythmic alterations of standard ballad melodies for expressive purposes. Without altering a single pitch, these performers take melodies which are so well-known as to be cliched and make them sound fresh. The primary technique is that of extensive rubato, where the melody is played at a varying tempo against the steady rhythmic background of a rhythm section (typically drums, bass, and piano). This results in a displacement of the melodic note relative to its nominal position against the background. Types and degrees of rhythmic variation are shown to depend on motivic structure (parallel motives are performed in similar ways, preserving the originals proportional rhythmic structures), metric emphasis (downbeats are treated differently than other locations), and harmonic structure (melodic tones belonging to the underlying harmonies are rhythmically displaced in a more extreme manner than tones outside the harmonies). Consideration is given to the way in which very familiar tunes are treated compared with less-familiar melodies (for example, ``My Funny Valentine'' compared with ``Goodbye, Pork-Pie Hat''), leading to discussion of the role of long-term memory in expressive performance. Analyses are given of several melodies, taken from commercial recordings by master improvisors.