Electrification was first used for lighting; there wasn't any need for a receptacle standard because there were no electric appliances to plug in. So the first electric receptacle standard (such that it was) was the Edison Socket as found on standard Incandescent (Halogen/LED) bulbs today.

The disadvantages of the Edison Socket (mostly that it exposes a live conductor to anyone with fingers but also that it is time consuming and annoying to screw a cord into) quickly led to introduction of several two conductor push in plug/receptacles. The most common were today's two vertical blades (patented by Hubbell in 1910 (equal width) and 1916(polarized) followed by two horizontal in line blades and a distant third were two round pins. There were other minor options like many combinations of straight and slanted blades. Then as now these "standards" competed and many receptacles were offered that allowed for two different plug styles (either vertical and horizontal blades or one of the flat blades combined with round pins). None of these designs incorporated a third "ground" (actually a bonding) connection because the advantage of grounding one of the conductors at the service hadn't been recognized yet. Around 1915 the two parallel vertical flat prongs (Current NEMA 1-15) won out when Bryant figured out how to make a cheap receptacle to match and by the mid 20s all devices came with that style plug. Eventually it was mandated by code for 120V/15A circuits.

The ground pin was invented in 1928 and receptacles accepting a NEMA 5-15 plug (the current standard) were mandated by the National Electrical Code to be solely installed in residences in 1962. Very few homes had three prong receptacles before 1960.