The difference between a T2 and a T3 is the size (more precisely the width) of the tubing. T2 is 2/8 of an inch, T3 is 3/8 of an inch, and so on and so forth. The larger (thicker) tubes used in offices are usually T5 (5/8 of an inch), T8 (1 inch) or T12 (1.5 inches). The rule of thumb is the thinner the tube the more efficient the lamp, although other factors can affect the efficiency. For CFLs, the T2's allow manufacturers to make smaller bulbs often marketed as "micro minis".
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