Happy Couple

Why do we have pairs of scissors? Since that's how they've always been known, why do we care why we call them a pair when one is no good on its own? The same thing applies to our trousers, just wearing the one would be daft, just showing one knee, ugly veins you could see and I don't think I'd cope with the draught. The same thing applies to my spectacles, another I couldn't let pass, since one on its own as a monocle it's known so why not just call it a glass? I asked my good Lady's opinion, she said 'oh, that's quite plain to see, as a pair we get on, and the jobs all get done but you'd be useless without me'.