Semi-detached housing consists of pairs of houses built side by side as units sharing a party wall and usually in such a way that each house's layout is a mirror image of its twin. this style is sometimes colloquially called a duplex; elsewhere, however, "duplex" refers to a building split into two flats/apartments (one above the other). The style is usually referred to in the mid-Atlantic (particularly Philadelphia) as a twin.5 W* G# e% ~, i" J( E5 r/ F( l9 @