How did alphabet letters get the names "upper" and "lower" case?

When printing presses were used, they comprised of small metal letter molds that were arranged together to form the desired text. "Lower case" letters were stored together in a literal case placed at an easily accessible location, as they were the most frequently used letters. "Upper case" letters were stored in a higher location as they were used less frequently. It appears it was really that simple!

https://www.mcgill.ca/oss/article/did-you-know-history/why-it-called-upper-and-lower-case