Daniel Christlein to Margaret Mikulska:

>>(...) The motif itself comes from plainchant, although I can't recall
>>right now from where exactly - I think it's one of the Gregorian Gloria
>>movements.
>
>(...) Of course, the appearance of these notes in various works by
>various composers could have been a coincidence, but the plainchant origin
>would certainly explain its ubiquity (not unlikely, then, as Steve Schwartz
>suggested, that composers would have been regularly exposed to it while
>studying counterpoint).

I found the famous motive in a book of counterpoint exercises by Cherubini
(which I assume he took from previous sources).  It appears as a "cantus
firmus" (i.e.  a given voice) under two versions:

a) c-d-f-e-g-f-e-d-c

b) c-d-f-e-a-g-f-e-d (this is almost the melodic skeleton of Mozart's 41
finale theme).

Pablo Massa
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