Harvey Hubbell Electric Co. was one of the earliest inventors of the electric plug. They have several patents for plug connections that eventually leads to the plugs we use today. Back From Left to Right: 4 slightly different embossings Tandem spade type plug adapters Patent 774251 in 1904,Next is 2 different embossings with the T slot or Tandem and parallel plug spades. Front Row: Hubbell T slot and Tandem combo. On some aplications one plug was made a t slot so it would be plugged in the right way but you could also use just a regular Tandem plug. Next is a Parallel plug adapter in black glaze. This was under patent 1064837. This was the pre runner to the polarized plugs we use today. Polarized plugs ensure that the wires are running in the correct electrical path with the outlet. The currents path runs through the small spade (Hot), and out the larger spade (Neutral). This eliminates a reverse current where it runs in the oposite direction and could cause damage or a fire. Polarized plugs were under Patent 1180648 in 1915. Next is a wall mount T slot Polarized combo made by Arrow Electric Co. Lastly is a angle slot wall mount plug made by Hubbell. It was covered under patent 1179728 and was usually used with larger appliance applications.