Another lesson learned at Colonial Williamsburg. =) Well, I'm pretty sure I'd learned this before, but not with a nice visual handy...
So since the mid 1800s, people have referred to capital letters as upper case and small letters as lower case. This is a direct borrow from printers' type cases, where they keep the metal letters with which they build their work. Since small letters are used far more often than capitals, these were stored more handily in the lower case. Capitals, which are used rather sparingly in comparison, were kept in the harder-to-reach upper case.
The simple names (upper and lower) for the type case have been used since the 1500s. I'm a bit surprised it took 300 years for the names to be transferred to the letters kept in the cases!
Interestingly, setting type was the job of the lowest (and generally shortest, ha ha) apprentice, so younger boys learning to be a printer might have a hard time reaching those capitals at all. (Now what's the excuse of my 9-year-old for hating to use them when writing with a pencil? That's another question altogether...)