PC floppy disks - technical definition

Until 1995, there were two kinds of floppy disks routinely used in a PC: the 5.25" disk, housed in a square, flexible envelope, and the 3.5" disk in its rigid plastic case. Today, 5.25" disks are obsolete, and the 3.5" floppy is the standard. Due to its small capacity, many computers come without floppy drives.

5.25" Diskettes
The first floppy, the low-density 360KB 5.25" diskette, was widely used as the distribution medium for software even after the high-density 1.2MB drive came out in 1984. The high-density drive also reads and writes the low-density format.

3.5" Diskettes
The 3.5" floppy was introduced in a low-density 720KB version on IBM's Convertible laptop. Capacity was doubled to 1.44MB with the PS/2 line. The high-density drive also reads and writes the low-density format. You can tell the difference between the 720KB and 1.44MB disks. Looking at it from the label side with the aluminum slider at the bottom, the 1.44MB disk has a hole in the upper left corner, while the 720KB disk does not.

For a while, IBM included an extra-high density 2.88MB floppy drive on selected models that was compatible with 1.44MB diskettes. The format never caught on.

 Floppy Disk Formats

  720KB   3.5"  DS/DD  Low density (Double Density)
 1.44MB   3.5"  DS/HD  High density
 2.88MB   3.5"  DS/ED  Extra-high density (IBM)

  360KB  5.25"  DS/DD  Low density (Double Density)
  1.2MB  5.25"  DS/HD  High density

  DS stands for double sided.