Why is it called case "folding"?

[Roger L Costello]

Hi Folks,

I read [1] that this is what case folding is:

Case folding is the process of making two texts which differ only in case identical for comparison purposes, that is, it is meant for the purpose of string matching.

I understand the use of the word "case," as in uppercase and lowercase. 

I don't understand the use of the word "folding." Folding? Huh? I fold my towel. I fold my shirt. I don't fold my case (whatever that means). 

Why is it called case "folding"? Who came up with that term?

/Roger

[1]  https://www.w3.org/TR/charmod-norm/#definitionCaseFolding