Notation of Interval Classes
The unordered pitch class interval i (a, b) may be defined as
where i <a, b> is an ordered pitch class interval (Rahn 1980, 28).
While notating unordered intervals with parentheses, as in the example directly above, is perhaps the standard, some theorists, including Robert Morris (1991), prefer to use braces, as in i {a,b}. Both notations are considered acceptable.
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