pictures are always good. I believe OP is speaking about a 15/20 amp 120 volt recep:
that is a
receptacle designed to be able to accept a 15 amp 120 volt plug or a 20 amp 120 volt plug..
a 20 amp 120 volt (nema 6-20) recep (r) and plug (p) have the neutral slot in a horizontal position.
In contrast to a 20 amp 240 volt recep would have the slot on the left in the pic horizontal