(Constant Linear Velocity) Rotating a disk at varying speeds. By changing speed depending on which track is being accessed, the density of bits in each track can be made uniform. It allows the data transfer rate to be constant and the outer tracks to hold more data than the inner ones. CLV is used on optical media such as CDs and DVDs. In practice, the rotation does not change precisely from every track to the next. A data buffer provides some flexibility for changing speed across some number of tracks. "Zone CLV" (Z-CLV) breaks the disk into several zones and changes the speed within each zone rather than uniformly across the entire platter. Many CD-ROM drives use Z-CLV with three zones. DVD-RAM drives may have as many as 24 or more zones. Contrast with CAV, which rotates the platter at the same speed.