Flippy disk

A flippy disk (sometimes known as a "flippy") is a double-sided 5¼" floppy disk, specially modified so that the two sides can be used independently (but not simultaneously) in single-sided drives. Use of "flippy" disks was most common during the 8-bit home computer era of the early-to-mid 1980s.
Generally, there were two levels of modifications:
* For Disk Operating Systems that did not use the index hole in the disk to mark the beginnings of tracks, the "flippy" modification required only a new write-enable notch to be cut. For this purpose, specially designed single-square-hole hole punchers, commonly known as disk doublers, were produced and sold by third-party computer accessory manufacturers. Many users, however, made do with a standard (round) hole puncher and/or an ordinary pair of scissors for this job.
* For disk operating systems that did use index sync, a second index hole window had to be punched in both sides of the jacket, and for hard sectored formats, an additional window must be punched for the sector holes. While cutting a second notch was relatively safe, cutting additional windows into the jacket was at great peril to the disk within.
A number of floppy disk manufacturers produced ready-made "flippy" media. As the cost of media went down, and double-sided drives became the standard, "flippies" became obsolete.
Special cases
* At least one manufacturer created a diskette drive for the TRS-80 platform with two index read sensors so that the disk could be flipped over without the necessary index hole punch. Data written with these rare drives complicates conversions today for retrocomputing archivists due to the sector offset skewing: the sectors start earlier than they would using the standard index hole placement.
Two versions on one disk
* During the time of single sided floppy disks some software manufacturers would use the two sides of one floppy disk to store two different copies of the same program. Each side would contain one copy of the program designed to be run on a different computer architecture. For instance several games were released that contained a Commodore 64 version on one side and an Apple II or an Atari 8-bit version on the other side of the disk. These releases were short lived however. When double sided floppy drives became common on gaming platforms this technique soon disappeared.
* As an easter egg, the Apple II version of Karateka, a 1984 computer game by Jordan Mechner, contained a second version placed on the flip side of the disk. If one put the disk into the drive upside-down, the game played identically to the first side, except that the game was displayed vertically flipped.

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