Choosing Halogen Bulbs
Mains Voltage - GU10
The GU10 has two studs at the bottom of the bulb and a twist-lock action to insert and remove from the fitting. The reflector will generally measure 50mm across the face of the bulb, but other sizes are also made. It is a 240V mains halogen and is quite different from the 12V versions. The 10 in GU10 means there is a 10mm gap between the centres of the two studs.
Mains Voltage - G9
The G9 has two flat pins at the bottom and simply pushes into the fitting. The buld is a 'capsule' only and is very compact in size.
Low Voltage MR16 / GU5.3
The MR16 has 2 sharp pins on the bottom of the bulb and is simply a push fit into the fitting. This is a 12V lamp which requires a transformer and quite a bit larger than the MR11. It measures 50mm across the face of the bulb. The 5.3 in GU5.3 means that there is a distance of 5.3mm between the two pins.
Low Voltage MR11 / GU4
The MR11 has 2 sharp pins on the bottom of the bulb as well and is also a push fit into the fitting. It is a 12V lamp which requires a transformer, but what makes the difference between this and the MR16 is the width of the bulb. This measures 35mm across the face of the bulb. The 4 in GU4 means that there is a distance of 4mm between the two pins.