If I thought about it at all, I assumed that upper case and lower case were just two different cases (options) for big, or small, letters. You might therefore assume that these terms have been with us since the invention of writing, or at least writing in two sizes.
These terms came into being with the invention of moveable type and the printing press (1450 A.D.). Typesetters would pick letters from a large case organized by letter. And — you guessed it — capital letters were in the “upper case” and the rest were in the “lower case.” The terms referred to their physical location, which quickly became convention, because then a typesetter from one press could quickly adapt to another press. Yet now the terms are so generic that they are used even in handwriting instruction. The printing press’s influence echoes down the ages!
Notice the upper-case letters had slots of equal size, while the lower-case letters (more often used) had slots proportional to their frequency of use (in English). This is what you’d need when setting a single line of type.
There were already existing terms for the two cases. Capital letters were referred to as “majuscules” and small letters were “minuscules.” But such was printing’s influence that the jargon of the trade has spread out to general use. Also, scripts that have two sizes, like this, are referred to as “bicameral” scripts (just like bicameral government!).
I learned about this in “The Coming of the Book: The Impact of Printing 1450-1800″ by Febvre and Martin, which I am reading for a library school course on the history of books and libraries. This book contains other interesting tidbits, like the fact that once the printing press got going, it was very productive; skilled teams could produce a sheet every 20 seconds. Further, there’s a sordid and fascinating story behind Gutenberg and his associates Fust and Schoeffer, who took over his printing press just as he was finalizing the process, because Gutenberg defaulted on a loan; as a result, none of the books printed with his press bear his name as the publisher, although the history books have given his name full credit.
Nowadays even fonts displayed digitally continue the use of “upper” and “lower” case to distinguish these two components of the English script. The very term “font” is also an echo of early printing press technology developments, as it comes from “fondue” which means something that has been melted; early fonts were cast in metal at a type foundry. Can we imagine “tweet” or “text” or “facebook” persisting in our vocabulary for a similar span of more than 500 years?