The original “X” that is used to denote reading and writing speeds in optical drives of all types refers to the speed of the first drive of that type. So a 52X CD drive can read or write data 52 times faster than the original CD drive, which means at about 150 x 52, or 7800, Kilobytes per second. The notation seen on optical drives refers to the speeds during different operations. For example, a CD-Writer with the 52X24X40X notation can read CDs at 52X, erase CDs at 24X, and write CDs at 40X.