"PAR" is an acronym for "Parabolic Aluminized Reflector", this refers to the shape and design of the light bulb. The "30" in the PAR-30 or PAR30 refers to the dimensions of the FACE of the bulb (or lamp) in eighths of an inch; thus a PAR30 is 30 eighths of an inch in diameter across the face of the bulb, 3.75" (95.25mm) Maximum Outer Diameter. PAR30 LONG NECK (or commonly referred to as LN or PAR30FL/LN or PAR30SP/LN) has a much longer neck than a standard PAR30 or PAR-30. Typically PAR30 Long Neck bulbs have a maximum overall length of 4.69" (119.126mm). "PAR" bulbs are usually but not exclusively halogen bulbs by design.