PC-DOS 2.88M format
The PC-DOS 2.88M format (3 1/2", double sided, extended density) was the highest-capacity floppy disk format to be used on a PC platform. It had 80 tracks per side, with 36 sectors per track, and 512 bytes per sector. Data was stored with MFM encoding, and generally used the FAT12 file system. The disk turned at 300 RPM. The capacity was twice that of the PC-DOS 1.44M format, but special extended-density disks were needed, which were never manufactured in the quantities of other formats since this format didn't catch on. Introduced on high-end IBM PS/2 models, these disks (along with the computers they ran on) failed to gain a foothold in the market.
This format is distinct from other formats also labeled "extended density" by IBM which can get up to 1.86 MB of data on a standard high-density floppy disk (see Wikipedia article).
3 1/2" disks are actually 90 mm wide, but are almost universally referred to as "3 1/2 inch" disks even in countries that use the metric system.