Collating Elements vs. Characters
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  POSIX generalizes the notion of a character to that of a collating
element.  It defines a "collating element" to be "a sequence of one or
more bytes defined in the current collating sequence as a unit of
collation."

  This generalizes the notion of a character in two ways.  First, a
single character can map into two or more collating elements.  For
example, the German "es-zet" collates as the collating element `s'
followed by another collating element `s'.  Second, two or more
characters can map into one collating element.  For example, the
Spanish `ll' collates after `l' and before `m'.

  Since POSIX's "collating element" preserves the essential idea of a
"character," we use the latter, more familiar, term in this document.