A type "T" halogen light bulb is a halogen bulb bearing a 'tubular" shape. Unlike other bulbs that contain argon or nitrogen gases in the envelope surrounding the filament, these longer-lasting bulbs bear gases from the halogen group that blend effectively with the tungsten atoms for increased output.
Some common "T" bulbs are the T-4, T-6, T-7, T-8 and T-10, with the number designating the diameter of the bulb in eighths of an inch. For example, a T-6 bulb has a diameter of 0.75 inches, which is 6/8 of 1 inch. Other bulb shapes include A (standard), B (bullet), C (candle), F (flare), G (globe), R (reflector) and S (sign).