Set consists of a 4 pin tube base with a mating 5 pin socket top. Very handy for tube subsitutions; replace an 'antique' triode with a more available and cheaper 5 pin based tube 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.