Originally Posted by FBinNY

Discounting vintage and now rare sizes, and the uncommon heavier spokes, spokes come in only 2 sizes as far as thread is concerned. 14g (0.80", 2mm) and 15g (0.72", 1.8mm). Both are threaded 56tpi. Likewise nipples only come in those two thread sizes. So it's no big deal to identify spokes by the diameter of the wire just beyond the thread, and to identify nipples using a 2mm spoke a a go/nogo gauge.