A "flippy disk" is a double-sided 5 1/4" floppy disk, specially modified so that the two sides can be used independently (but not simultaneously) in single-sided drives.

For operating systems that do not use the index hole in the disk to mark the beginnings of tracks, the "flippy" modification requires only a new write-enable notch to be cut. For those that do use index sync, however, a second index hole window must 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.