Disk density
Disk density is a capacity designation on magnetic storage, usually floppy disks. Each designation describes a set of characteristics that can affect the areal density of a disk or the efficiency of the encoded data. Such characteristics include modulation method, track width, coercivity, and magnetic field direction. 8-inch media. "Single density" (SD or 1D) describes the first generation of floppy disks that use an iron oxide coating. Floppy drives utilize 300-oersted write heads, FM encoding, and a track width of for a density of 48 tracks-per-inch (tpi) and 5876 bits-per-inch (bpi). "Double  density" (DD or 2D) doubles capacity over SD by replacing FM encoding with an improved line code, such as modified frequency modulation (MFM), modified modified frequency modulation (M²FM), FM/MFM or group coded recording (GCR). 5¼-inch media. "SD" ("1D") and "DD" ("2D") designations were generally identical to those of 8-inch disks. "Quad density" (QD or 4D) doubles capacity over DD by narrowing the width of tracks to for a density of 96 tpi. Some manufacturers (Micropolis, Tandon, Micro Peripherals (MPI), Teac) used a track density of 100 tpi for quad-density drives, which were incompatible with 96 tpi models . "High density" (HD) improves capacity by utilizing a 96 tpi track density in conjunction with improved cobalt disk coating and stronger 600-oersted write heads, allowing 9646 bpi to be written. 3½-inch media. "Double density" (DD) 3½-inch disks use an iron oxide coating, just as with 5¼-inch DD/QD disks. However, drives utilize stronger 670-oersted write heads and a narrower track width of for a density of 135 tpi and 8717 bpi. "High density" (HD) 3½-inch disks switch to a cobalt disk coating, just as with 5¼-inch HD disks. Drives use 700-oersted write heads for a density of  17434 bpi. "Extra-high density" (ED) doubles the capacity over HD by using a barium ferrite coating and a special write head that allows the use of perpendicular recording. "Triple density" (TD) triples the capacity over ED by tripling the track density and improving other parameters. The drives used longitudinal recording.