the concept is concentric, hollow, cylindrical slices through the physical disks (platters), collecting the respective circular tracks aligned through the stack of platters. the number of cylinders of a disk drive exactly equals the number of tracks on a single surface in the drive. it comprises the same track number on each platter, spanning all such tracks across each platter surface that is able to store data (without regard to whether or not the track is "bad"). cylinders are vertically formed by tracks. in other words, track 12 on platter 0 plus track 12 on platter 1 etc. is cylinder 12.