Acorn double density 3 1/2" disk
The Acorn double density 3 1/2" disk was used with Acorn computers, a line of computers from the UK in the 1970s through 1990s, which included the popular BBC microcomputer, and eventually led to RISC OS. It succeeded the Acorn double density 5 1/4" disk, and was succeeded by the Acorn high density 3 1/2" disk.
The disks were double sided, with 80 tracks per side and variants having 16 sectors of 256 bytes, and 5 sectors of 1024 bytes, for a total capacity of 640 or 800 kilobytes respectively. MFM encoding was used.