"Upper" & "lower case" stem from #earlymodern printers having thus sorted their letters in a wooden case. #emthings pic.twitter.com/AWeXHtVUxl
— Alexi Baker (@AlexiBaker) December 3, 2014
The ensuing Twitter conversation goes on to reveal that "mind your p's and q's" and "cliché" both have share their origins with movable type printing presses. Neat!