Single Sided, Double Density

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SSDD originally referred to Single Sided, Double Density - a format of (usually 5¼") floppy disk which could typically hold 35-40 tracks of nine 512-byte (or 18 256-byte) sectors each. Only one side of the disc was used, although some users did discover that punching additional holes into the disc jacket would allow the creation of a "flippy" disc which could be manually turned over to store additional data on the reverse side.

Single-sided disks began to become "obsolete" soon after the introduction of the original IBM 5150 PC in 1981, which used 360Kb double-sided double-density drives. Ironically this same year, Commodore released a floppy disk system that could store 1MB of data but it was not well received in part because their users felt that it was overkill.