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. Data was stored with MFM encoding. The disk turned at 300 RPM.
These disks were generally used with FAT12 file systems under the MS-DOS or PC-DOS operating system.
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.