A Dictionary of Music and Musicians/Falsetto
FALSETTO. The voices of both men and women contain two—or, as defined in the 'Méthode du Chant du Conservatoire de Musique,' three—registers, viz. chest voice (voce di petto); head voice (v. di testa); and a third which, as being forced or non-natural, is called by Italians and French falsetto or fausset, or 'false' voice. The limits of these are by no means fixed. In every voice identical notes can be produced in more ways than one, and thus each register can be extended many degrees beyond its normal limits. But it is all but impossible for a singer to keep both first and third registers in working order at the same time. The male counter-tenor, or alto voice, is almost entirely falsetto, and is generally accompanied by an imperfect pronunciation, the vowels usually partaking more or less of the quality of the Italian u or English oo, on which the falsetto seems to be most easily producible.
The earliest mention of the falsetto in musical Europe is in reference to the Sistine Chapel, where Spaniards exceptionally gifted with this voice preceded that artificial class to whom since the 16th century alto and even soprano parts have been assigned.