R-12 Shaped miniature bulbs range in voltage between thirteen (13.0) and thirty eight (38.0) volts. The letter R-12 denotes that the bulb is REFLECTOR shaped and that the glass envelope has a Maximum Outer Diameter (M.O.D.) of twelve eighths of an inch or 1.55" inches which is 39.44 millimeters. All bulbs in the R-12 category have a Maximum Overall Length (M.O.L.) of 2.63" inches which is 66.802 millimeters. All R-12 miniature bulbs have either a C-8 or a CC-8 Filament Design.