Upper case and lower case letters.
We use them daily but don't give a second thought to what it actually means.
Back in the early days of the printing press, each letter was individually placed in a frame to form the page. Then all the letters were inked and pressed onto paper.
The individual letters were stored in boxes called cases. The capitalized letters were store in the upper case and the non-capitalized letters were stored in the lower case. This storage method became common place and we still use the terms uppercase and lowercase letters today.