The 3.5-inch microfloppy disk was a magnetic disk format for data storage, originally introduced by Sony in 1982. An improved design was introduced by the Microfloppy Industry Committee in 1983, with a single-sided disk and a capacity of 360 KB. A double-sided disk was introduced in 1984 (with a capacity of 720 KB).
High Density (HD) 1.44 MB disks, recognisable by a second hole in the opposite corner to the write-protect notch and a HD logo, were introduced in 1987.
The 3.5-inch disk was housed in a semi-rigid case, with a metal shutter to protect the disk. Because they were not quite square, it was not possible to insert a disk the wrong way round.
Macintosh computers used the same disks as other systems, but with different capacities due to varying the disk rotation speed with the arm position. This allowed single-sided floppies to provide 400 KB (twice for double-sided).
The 3.5-inch HD (High Density) microfloppy quickly become almost universally used on PC and Macintosh hardware.