CLHS: Section 19.2.2.1.2.2

19.2.2.1.2.2 Common Case in Pathname Components

For the functions in Figure 19-2, a value of :common for the :case argument that these functions should receive and yield strings in component values according to the following conventions:

* All uppercase means to use a file system's customary case.
* All lowercase means to use the opposite of the customary case.
* Mixed case represents itself.

Note that these conventions have been chosen in such a way that translation from :local to :common and back to :local is information-preserving.