T12 fluorescent light bulbs are a type of fluorescent lamp that has a diameter of 1.5 inches (12/8 inches). They are commonly used in commercial and industrial settings, as well as many residential applications. T12 lamps are being phased out and replaced by more energy-efficient T8 and T5 lamps. These T12 lamps have been often replaced by more energy-efficient LED T8 and various other light sources.