PC-DOS 1.2M format
The PC-DOS 1.2M format (5 1/4", double sided, high density) was the high-density counterpart of the PC-DOS 360K format for 5 1/4" floppy disks, storing over 3 times as much data due to use of a different media surface capable of handling a higher density of data. It had 80 tracks per side, with 15 sectors per track, and 512 bytes per sector. Data was stored with MFM encoding. The disk turned at 360 RPM. The high-density drives supporting this format were introduced in the mid-1980s with the IBM AT, and were commonplace by the end of the decade.
These disks were generally used with FAT12 file systems under the MS-DOS or PC-DOS operating system. High-density disk drives could handle both the new 1.2 MB format and the old 360K format, though 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; by the 1990s these were usually high-density drives supporting the 1.2M and 1.44M formats. 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.