# LED size nomenclature

Round LEDs are often indicated by "T-1" for 3mm LEDs or "T-1 3/4" for 5mm LEDs. What does the "T" mean, and how should I read other indications like this when I encounter them? "T" doesn't seem to fit neither mm nor inches.

So T-1 is 1/8 inch, or (about) 3mm, T-1 3/4 is 1.75 $\times$ 1/8 inch, or (about) 5mm. A 10mm LED is T-3 1/4.