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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.

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From this page:

The T-1-3/4 nomenclature indicates the shape and size of that particular device. The T or various other descriptions (S, G, PR, TL, etc.) indicates the envelope shape of the device [...]. The 1-3/4 indicates the envelope size in eighths (0.125) of an inch. An example is a T-1-3/4 lamp has a 0.219 diameter (1.75 X 0.125 = 0.219).

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.

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