PC-DOS 360K format
The PC-DOS 360K format was a very common floppy disk format in the 1980s, used on IBM PCs and compatibles. It replaced earlier 160K, 180K, and 320K formats using either fewer sectors, single-sided disks, or both, with a new format getting the most out of a double-sided, double-density 5 1/4" floppy disk. It had 40 tracks per side, with 9 sectors per track, and 512 bytes per sector. The disk turned at 300 RPM.
These disks were generally used with FAT12 file systems under the MS-DOS or PC-DOS operating system, but (far more rarely) were used with other filesystems too (e.g CP/M-86). The sector headers, etc, were based on IBM 3740 format, albeit with MFM encoding (double density) replacing the original FM encoding (single density).
After being a commonplace format for most of the 1980s, this format declined in favor of the high-density PC-DOS 1.2M format and the 3 1/2" PC-DOS 720K format and PC-DOS 1.44M format. (There were compatibility issues in reading 360K disks on low-density drives after they were written to with a high-density drive, even though the writing is done in an emulation of the old format, due to the different drive head on the newer drives. The high-density drive heads were smaller, and the data written by them might not be picked up correctly by the larger low-density heads, particularly if the new data was overwriting data stored earlier using large-headed drives, which might not be completely overwritten.)
In the late '80s and early '90s, it was common for desktop PCs to have both 5 1/4" and 3 1/2" disk drives in order to be compatible with all software and data, which might be distributed on either format. Often the 5 1/4" drive was drive A, and the 3 1/2" one was drive B. Later PCs, however, were more likely to have only a 3 1/2" drive, set up to respond to both drive letters. Eventually, PCs stopped having floppy disk drives altogether as other data storage and transfer media took over.
