PC-DOS 180K format
The PC-DOS 180K format was one of several low-capacity 5 1/4" disk formats used on IBM PCs and compatibles in the early days of PC-DOS before the PC-DOS 360K format became the standard. It used a single-sided, double-density disk with 40 tracks 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, 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). This format had one more sector per track than the PC-DOS 160K format, and was in the same format as the PC-DOS 360K format except using only one side of the disk instead of both sides.
As with most single-sided disk formats, users often flipped the disks over to double the storage capacity, which required cutting a write-enable notch on the opposite side from the standard one. Some disks were manufactured with a second notch to cater to this use, though disk manufacturers tended to discourage double-sided use by claiming the reverse side wasn't properly certified for data (despite the fact that different single-sided formats on different platforms actually used different sides of the media) and that flipping the disks caused dust that builds up on the disk to get into the disk drive.
