This is a protective and durable case for RFID key cards. This design came about when a much-used key card (which was kept on a shared keyring) kept having pieces break off of it from flexing during carrying and use. This case has served its purpose well for years and has proven its durability many times over. The key card can still be used without removing it from the case and is designed to be essentially a permanent addition to an RFID key card. More specifically, this is designed for RFID key cards with a particular form factor which I've encountered from a few different brands, for example Honeywell. They tend to be the same size as a US driver's license, slightly tapered on one side to better fit in a wallet, with a keyring hole on one end. If this describes your key card, there's a good chance it will work with this design. However, if you need to modify the design for your particular key card, the source FreeCAD model is included.