▁the ▁repet ition ▁and ▁development ▁of ▁a ▁number ▁of ▁pitch ▁and ▁rh yth mic ▁groups ▁described ▁by ▁Carter ▁as ▁" char acters ". ▁P itch ▁coh er ence ▁is ▁achieved ▁by ▁concentr ating ▁the ▁material ▁on ▁three ▁hier arch ically ▁prominent ▁ch ords , ▁which ▁increase ▁in ▁importance ▁as ▁the ▁quart et ▁progress es . ▁Two ▁of ▁these ▁are ▁t et rach ords , ▁the ▁third ▁a ▁hex ach ord . ▁In ▁Carter ' s ▁Harm ony ▁Book , ▁the ▁four - note ▁ch ords ▁are ▁numbers ▁ 1 8 ▁and ▁ 2 3 , ▁and ▁the ▁six - note ▁ch ord ▁is ▁number ▁ 3 5 . ▁ ▁The ▁quart et ▁represents ▁two ▁simultane ous ▁" cre ative ▁processes ". ▁In ▁performance , ▁the ▁mus icians ▁appear ▁to ▁be ▁first ▁trying ▁out ▁and ▁re he ars ing ▁musical ▁ideas ▁( in ▁the ▁introduction ▁and ▁inter l udes ), ▁and ▁then ▁playing ▁them ▁( in ▁the ▁even - number ed ▁movements ). ▁At ▁the ▁same ▁time , ▁the ▁sketch es ▁show ▁that ▁this ▁mirror s ▁the ▁composer ' s ▁compos itional ▁process ▁as ▁he ▁worked ▁out ▁small ▁sections , ▁not ▁writing ▁linear ly ▁from ▁beginning ▁to ▁end , ▁but ▁by ▁assemb ling ▁small ▁phr ases ▁together ▁and ▁pol ishing ▁them . ▁ ▁The ▁strong ▁characters ▁of ▁the ▁Quart et ' s ▁materials ▁emer ge ▁as ▁a ▁result ▁of ▁the ▁conflict ▁and ▁resolution ▁of ▁rh yth mic ▁art ic ulation ▁created ▁during ▁metric ▁mod ulation . ▁Through ▁the ▁concept ▁of ▁the ▁" time ▁screen