The 12mm M3 metal female-female spacer is a type of hardware used to provide a specific distance between two components in a mechanical assembly. The spacer is made of metal and has a diameter of M3, which is a standard screw thread size. The spacer is 12mm in length, providing a specific distance between the components it is separating. The spacer has female screw threads on both ends, which allows it to be easily attached to components with male screw threads. The female-female design provides a secure and stable connection, making it ideal for use in electronic, mechanical, and other applications where a specific distance between components is required.