Apel, Willi.
The notation of polyphonic music : 900-1600 / by Willi Apel
. - Cambridge, Massachusetts : The Mediaeval academy of America, c1961 . - 5th ed. rev. and with commentary . - XXV, 464 p., [15] c. di tav. : es. mus., ; 26 cm ( The Mediaeval academy of America
; 38 )