G4 socket: G indicates the type of lamp holders are a plug-in, followed by numbers indicate light foot hole center distance of 4mm. MR16 lamp holder: the multi-faceted reflector (lamp cup), followed by numbers, which means that the lamp cup diameter (unit is 1/8 inch), MR16 diameter = 16 × 1/8 = 2 inch ≈ 50mm, MR11 diameter = 11 × 1/8 = 2 inch ≈ 35mm.
