Linked from
The 45 pages that link to Polyphony, each with the reason it gives.
MotetNarrower topic: The motet is defined by combining independently moving vocal parts.
Medieval musicNarrower topic: Its emergence marks a major expansion beyond single-line chant.
MadrigalNarrower topic: Madrigals build their vocal texture from interacting independent parts.
Messe de Nostre DameNarrower topic: Four vocal parts create the Mass's interwoven musical texture.
ShchedrykNarrower topic: Leontovych builds the song’s texture by layering independent vocal entries.