Apple II 13 sector disk
The Apple II 13 sector disk was the original floppy-disk format for the Apple II computer line, used with the Disk II drives released in 1978, and with versions of Apple DOS up through 3.2.1. It was a 5.25" disk (single-sided, soft-sectored, meaning that the index hole was ignored in finding sector locations) with 35 tracks each containing 13 sectors, storing 256 bytes of data in each sector for a total of 116,480 bytes (113.75K), using GCR encoding (of a type known as "5 and 3") to store the data. This format was later superseded by the Apple II 16 sector disk, used in Apple DOS 3.3, which had an increased storage capacity and required a ROM upgrade to the disk drive units to support it.
Three tracks of the disk were taken up by the DOS software (on bootable disks), and one more track with the catalog (directory) for the Apple DOS file system, leaving 100.75K for file storage.
While the disks were single-sided, users often punched an extra notch on the opposite side to permit the disks to be flipped over for use of the other side (the notch was necessary to prevent the disk from being seen by the drive as write-protected). This was done despite being discouraged by disk manufacturers, since the disks weren't designed to work well this way (only one side was certified for data storage, and also running it in both directions could cause debris to build up in ways unanticipated by the designers). However, some disk manufacturers eventually did produce double-notched disks for this use.
When the 16-sector format was introduced, a utility called MUFFIN was distributed on the system disk to transfer files from 13-sector disks into the new format. An "unofficial" utility called NIFFUM transferred files in the opposite direction. Another unofficial utility, "Advanced Demuffin", handled various sorts of nonstandard disks (particularly copy-protected ones) with different track/sector layouts.