▁music ▁notation ▁developed ▁in ▁religious ▁institutions , ▁and ▁the ▁application ▁of ▁notation ▁to ▁sec ular ▁music ▁was ▁a ▁later ▁development . ▁Early ▁in ▁the ▁period , ▁Greg or ian ▁chant ▁was ▁the ▁dominant ▁form ▁of ▁church ▁music ; ▁other ▁forms , ▁beginning ▁with ▁organ um , ▁and ▁later ▁including ▁cla us ula e , ▁conduct us , ▁and ▁the ▁mot et , ▁developed ▁using ▁the ▁chant ▁as ▁source ▁material . ▁ ▁During ▁the ▁ 1 1 th ▁century , ▁Gu ido ▁of ▁Are z zo ▁was ▁one ▁of ▁the ▁first ▁to ▁develop ▁musical ▁notation , ▁which ▁made ▁it ▁easier ▁for ▁sing ers ▁to ▁remember ▁Greg or ian ▁ch ants . ▁ ▁It ▁was ▁during ▁the ▁ 1 2 th ▁and ▁ 1 3 th ▁centuries ▁that ▁Greg or ian ▁plain chant ▁gave ▁birth ▁to ▁poly phony , ▁which ▁appeared ▁in ▁the ▁works ▁of ▁French ▁Notre ▁D ame ▁School ▁( L éon in ▁and ▁P é rot in ). ▁Later ▁it ▁evol ved ▁into ▁the ▁ar s ▁nova ▁( Ph ili ppe ▁de ▁Vit ry , ▁Guillaume ▁de ▁Mach aut ) ▁and ▁the ▁musical ▁gen res ▁of ▁late ▁Middle ▁A ges . ▁An ▁important ▁composer ▁during ▁the ▁ 1 2 th ▁century ▁was ▁the ▁nun ▁H ilde g ard ▁of ▁B ingen . ▁ ▁The ▁most ▁significant ▁sec ular ▁movement ▁was ▁that ▁of ▁the ▁trou bad ours , ▁who ▁arose ▁in ▁Occ it ania ▁( S ou thern ▁France ) ▁in ▁the ▁late ▁ 1 1 th ▁century . ▁The ▁trou bad ours ▁were