Funny you guys mentioned the 18mm and the 20mm sockets. A buddy of mine stopped over a few months ago and asked if he could borrow my 18mm and 20mm sockets. We went out to the garage to get them and it turns out I didn't have either size, but I do have a 17mm, a 19mm, a 21mm etc. I have no idea why I don't have the 18mm or the 20mm. Apparently they didn't come with the set I bought, not sure why, they just didn't, and for reasons I don't understand it never dawned on me that I didn't have those sizes.
Anyway, I went out and bought the 18mm and 20mm 6 point sockets just so I would have them on hand in case I needed them and they are sitting here right by my computer. My buddy didn't come back to borrow them so I guess he must have bought his own sockets. I guess I should take them out to the garage and put them someplace close by my metric sockets, not sure why I haven't done that yet either.