Set consists of an 'antique' 4 pin tube base with a mating 4 pin socket top. Very handy for tube subsitutions; replace an 'antique' rectifier with one pinout with another with a different pinout (e.g. 5Z3 to 80 etc.. My advice is to use Teflon insulated wire allowing enough extra lead length so you can move things around a bit if necessary. The top socket is best affixed to the the base with just a bit of Crazy Glue.