There are three main fluorescent tube groups, categorised as T5, T8 and T12. T12 technology was introduced back in the 1930’s, where as T8 came later in the 1980’s. Finally T5 was introduced in the 1990’s but took until the 2000’s to become recognised and widely used.
The “T” stands for tubular and the two-digits represent the diameter, hence T12 = 12/8 inches, T8 = 8/8 inches and T5 = 5/8 inches. Both T8 and T12 are equipped with the same G13 pins, whereas T5 uses the G5 pins. T5 tubes can fit onto T8/T12 lampholders with an aid of a retro-fit adapter kit.