Acorn double density 5 1/4" disk
The Acorn double density 5 1/4" 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 single density 5 1/4" disk, and was succeeded by the Acorn double density 3 1/2" disk and the Acorn high density 3 1/2" disk.
The disks had single and double sided variants. The single sided ones had variants with 40 and 80 tracks, each with 16 sectors to the track and 256 bytes per sector, for a capacity of 160 and 320 kilobytes respectively. The double sided variant had 80 tracks per side for a total capacity of 640 kilobytes. MFM encoding was used.