 FACT: In Max Dreyfus office, Oscar Levant while looking at Georges  score of "Swanee" hums a perfect forth (D down to A) and says, "A  diminished 9th." In the next scene Max Dreyfus also refers to the  uniqueness of a diminished 9th. In Swanee there is no diminished  9th. There are enharmonic equivalents ie. a C# melody against a Db  bass which sounds like a diminished 9th. It is acually a sharped  major 7th. The interval of a diminished 9th ie. (C up to Dbb or C#  up to Db)does not happen in Swanee. Both a diminished 9th and an  enharmonic equivalent will sound like a perfect octave when played  eight notes apart.