S1 has outgrown his 16" bicycle and so we went out and bought a 20" one yesterday. I had taken it out of the box and started to assemble it when I discovered that I needed an Allen key. Turned out that I had three sets of Allen keys -- in one set, I had 4mm and 5mm. In the other set, I had 7mm, 8mm and 10mm. The third set was English units -- 5/32, 3/16, 1/4 and 3/8-inch keys.
But the Allen key that I needed to assemble the bike? 6 mm. Of course Home Depot's not going to carry a single key. I'm going to have to buy yet one more set, this time one that includes a 6mm key.
Wikipedia, useful as always, offers: 4 mm keys are almost exactly the same size as 5/32", and 8 mm keys are almost exactly the same size as 5/16", which makes 4 mm and 8 mm preferred numbers for consumer products such as self-assembly particle-board furniture, because end users can successfully use an imperial key on a metric fastener, or vice versa, without stripping. Wish the designers of Schwinn bicycles would read Wikipidea.