Floppy disk

File Format
Name	Floppy disk
Ontology	Physical File Formats Floppy disk ; ;
	Some floppies, 5 1/4" and 3 1/2".

See also Filesystems, which are contained on Floppy Disks.

Formats for images of floppy disk data can be found at Disk Image Formats.

The main ways in which floppy disks could differ at a recording level:

The physical dimensions and physical layout – the most common sizes were 8-inch, 5¼-inch (officially the metric 130 mm, but in common parlance referred to by its inch approximation even in countries which normally use the metric system), 3½ inch (likewise officially metric 90 mm, but commonly known by the inch approximation). Note those are the sizes of the actual magnetic disk, the jacket/cartridge is larger. However, there were many other rare sizes.
The method of data encoding: the earliest floppies IBM 8-inch floppies used FM encoding. Capacity was doubled, without changing the disks physically, by switching to the more efficient MFM encoding. The original FM encoded disks were called "single density", and the MFM encoded "double density". IBM PC 5¼-inch and 3½ inch floppies used MFM encoding; the term "double density" was later extended to "high density" and beyond, but this time it referred to changes in the magnetic material and type of disk heads rather than an encoding change. GCR encoding was also popular, particularly with systems such as Apple IIs and Commodore 64s. Some early "double density" drives used M2FM encoding (most notably Intel ISIS-II and HP 9885) which was later replaced by the simpler MFM.
Magnetic recording material: more expensive media could store more data
Type of magnetic disk heads: more expensive magnets could store more data
Speed of rotation
Constant angular velocity (CAV) versus constant linear velocity (CLV)
Number of sides – early disks and drives were single-sided only and could only record on one side of the disk; later disks and drives were double-sided and supported recording on both sides. There were also "flippy disks" where a single-sided drive could be used to record on both sides of a disk by the user manually flipping the disk to access the other side.
Longitudinal magnetic recording versus perpendicular magnetic recording: traditionally floppies used longitudinal; 2.88 MB floppy drives doubled the capacity over 1.44MB by switching to perpendicular
Sector size: the earliest floppies had 128 byte sectors; IBM PC floppies normally had 512 byte sectors. IBM PC floppy controllers could support any power of 2 from 128 to 4096. Rarely, different tracks on a floppy could be recorded with different sector sizes, or even a single track with a mix of different sector sizes.
Gap length between sectors: reducing the gap length could squeeze more sectors on to the disk but at the risk of data loss
Format of track and sector headers, including CRC algorithms and any per-sector flags (e.g. IBM 3740 has a flag to mark each sector as "deleted", a feature carried forward into IBM PC floppy formats, but almost never used in them.)
Number of tracks per disk and number of sectors per track

A given combination of floppy disk controller, floppy drive, and disk, could generally support several different variations on the same basic format, but only within certain constraints – e.g. IBM PC floppies could be formatted with a non-standard sector size or number of tracks, but not with GCR or MMFM encoding (since the floppy disk controllers used in IBM PCs did not support those encodings).

The disk geometry, encoding, etc, is orthogonal to the filesystem – 1.44MB Apple Mac floppies were physically interoperable with IBM PC floppies, even though IBM PCs normally lacked software to read Apple's HFS filesystem; by contrast, the earlier 800KB and 400KB Apple Mac floppies were incompatible with IBM PC floppies, since IBM PC floppy drives could not physically read them, nor could those earlier Mac floppy drives physically read IBM PC floppies.