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Reference.org
Interval class
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<p>In <a href="/facts/Set_theory_(music)/JerBNFeX">musical set theory</a>, an interval class (often abbreviated: ic), also known as unordered pitch-class interval, interval distance, undirected interval, or "(even completely incorrectly) as 'interval mod 6'" (Rahn 1980, 29; Whittall 2008, 273–74), is the shortest distance in <a href="/facts/Pitch_class_space/kccbRRLo">pitch class space</a> between two unordered <a href="/facts/Pitch_class/40mqdJtN">pitch classes</a>. For example, the interval class between pitch classes 4 and 9 is 5 because 9 − 4 = 5 is less than 4 − 9 = −5 ≡ 7 (mod 12). See <a href="/facts/Modular_arithmetic/0wtRa5dU">modular arithmetic</a> for more on <a href="/facts/Modulo_operation/V5xPUluD">modulo</a> 12. The largest interval class is 6 since any greater interval <i>n</i> may be reduced to 12 − <i>n</i>. </p>