These key fobs operate at a frequency of 13.56MHz and are not compatible with other frequencies such as 125kHz. They are read only and not able to be re-programmed. Each key fob is pre-programmed and cannot be altered using a card writer. They are also compatible with the MIFARE Classic 1K Card (M1 Card) and can be used with a lock that uses M1 cards. These key fobs are suitable for use with a 13.56MHz RFID proximity access control system and ID management system, such as registering them as new keys for an RFID door lock. The key fobs are approximately 1.58*1.26*0.18 inch in size, made of ABS Plastic, and are black in color. The package includes 100 pieces.