Abstract: A movable fastener that permits, among other things, the interlocking of 3.5 inch flexible disk cartridges or "floppy disks." The fastener can project from the flexible disk cartridge when desired to fasten to another flexible disk cartridge. The fastener can be placed anywhere within the flexible disk cartridge where it will not interfere with the magnetic media, the shutter, the write protect hole and tab, the disk size indicator, or any other feature integral to the proper functioning of the flexible disk cartridge. Mortise and tenon, interference, and friction fit type fastener configurations are disclosed.