Also, an informal name for the 3"
disk drive used by
Amstrad on the
CPC, the
PCW and the
Sinclair Spectrum +3. Unlike the customised
5 1/4 inch disks mentioned above these were purposefully designed to be flipped over. They held a grand total of 350Kb - per disk, not per side.
The disks were encased in a hard plastic shell with an unusual dust cover mechanism located inside the shell, as opposed to the external dust cover on a 3.5 inch disk. Because it was inside the shell, it didn't actually protect the inside from dust, as every time it slid open whatever cruft had amassed on the cover was wiped onto the inner surface of the plastic case.
The disks themselves were roughly four times the price of 3.5 inch disks, and were apparently adopted by Amstrad purely because the company had aquired the assets of the company that made the drives at a knock-down price, and thus had a warehouse full of them.
The format died a death in the mid-90s, although it was a sickly child from the start. Amstrad wound up its home computer wing, the PCW gained a conventional disk drive, and the 3" format shuffled off into oblivion.