and some of them use the factory lugnuts to bolt the spacer on and there is lugs recessedin the spacer giving you enough lug to put the wheel on with. sort of like -l_ if that makes any sense.
those would be known as wheel adapters. one lug pattern that bolts to the hub like a wheel would, with the other set of studs sticking out of the 'spacer' part. you can get same/same or different patterns.