In (info "(coreutils) join invocation") If the input has no unpairable lines, a GNU extension is available;
(shouldn't that ";" be ":"?) the sort order can be any order that considers two fields to be equal if and only if the sort comparison described above considers them to be equal. For example: $ cat file1... OK, but perhaps in the previous paragraph give a mundane regular example of join usage, instead of the first example be this special exception.