The "U" style connector was invented by 3M® in the 1970's for communications applications. It eliminated the need to strip and twist the POTS (Plain Old Telephone) wires by using insulation displacement "U" shaped crimping contacts. Insert the wires, crimp the circle shut and you have a secure connection. Over time, a variety of wire sizes and tap types were created and color codes used to tell the wire sizes. The UY is a butt connector for 2 or 3 solid wires 19-26AWG. It is filled with a moisture resistant gel. Comes in three styles: UY (moisture-resistant seal), UY-2 (RUS listed) and UY2-D (dry). A similar product, the UR, handles a larger wire size.