FAT16

FAT16 (a variety of FAT, which stands for File Allocation Table) is a filesystem adapted from FAT12 at the time of the IBM AT computer, with more bits added per entry in order to support the increasing size of hard disks. It still had enough limitations that later revisions (by various device manufacturers) changed things in order to support more storage, such as using "logical sectors" bigger than the physical sectors so that the maximum number of addressable sectors in a 16 bit table entry contains more bytes. Ultimately, FAT32 increased the bit size yet again to support even larger devices.

Resources

 * Wikipedia
 * Microsoft EFI FAT32 File System Specification (which also discusses earlier FAT versions)