TRS-80 double density 5.25" disk
The TRS-80 double density 5.25" disk was a format used with TRS-80 Model I/III/4 computers. The TRS-80 single density 5.25" disk was the official format for that system, but third party manufacturers created double density drives to allow for an increased data capacity, and that eventually became an official upgrade. Drives were single-sided, but people often punched extra notches in them to make them "flippies" to use the other side, as with other platforms.
An enhancement to the TRSDOS operating system called DoubleDOS was used.
There were variants with 35 and 40 tracks as with the single density disks.