CD-R (Compact Disc-Recordable) is a digital optical disc storage format. A CD-R disc is a compact disc that can be written once and read arbitrarily many times. CD-R disks (CD-Rs) are readable by most plain CD readers, i.e., CD readers manufactured prior to the introduction of CD-R. This is an advantage over CD-RW, which can be re-written but cannot be played on many plain CD readers.