None, a cappella is defined as music without instrumental accompaniment.
“Cappella” literally means “chapel” in Italian. When the term was first coined, a cappella was a phrase that instructed performers to sing “in the manner of the chapel.” In modern sheet music, it simply means to sing without accompaniment.
The origin and creation of a cappella music is impossible to pin down. After all, cavemen humming to themselves were singing a cappella. What matters most, like languages, is when the music was written on paper (or stone). One of the earliest examples of sheet music was discovered on a cuneiform tablet dating back to 2000 B.C.
From what scholars can tell, it describes a piece of music written in a diatonic scale. Just recently, one of the earliest known scores for polyphonic music (music written with more than one vocal or instrumental part), written around the year 900 A.D., was discovered and performed at St John’s College, University of Cambridge.
The use of a cappella music gained popularity, especially in western music, largely in part to religious institutions. Christian churches predominantly performed gregorian chant throughout the medieval period and well into the renaissance period.
Read more about the history of a cappella at ThoughtCo.com.