, ▁was ▁completed ▁in ▁ 1 9 4 9 , ▁but ▁since ▁M atter ▁ins isted ▁on ▁asking ▁C age ▁to ▁compose ▁the ▁music , ▁the ▁team ▁had ▁to ▁wait ▁for ▁about ▁a ▁year ▁until ▁C age ▁returned ▁from ▁Europe . ▁The ▁piece ▁consists ▁of ▁three ▁sequences , ▁two ▁of ▁which ▁( numbers ▁ 1 ▁and ▁ 3 ) ▁are ▁for ▁prepared ▁piano , ▁and ▁one ▁( number ▁ 2 ) ▁is ▁for ▁t ape : ▁C age ▁recorded ▁the ▁sounds ▁of ▁Cal der ' s ▁studio ▁while ▁the ▁sculpt or ▁was ▁working ▁— m ob iles ▁b ump ing ▁into ▁one ▁another , ▁etc .— then ▁edited ▁the ▁resulting ▁two - hour ▁recording ▁in ▁order ▁to ▁produce ▁the ▁desired ▁sequence ▁of ▁sounds . ▁ ▁Two ▁Past or ales ▁Com posed ▁in ▁November ▁ 1 9 5 1 ▁( first ▁piece ) ▁and ▁January ▁ 1 9 5 2 ▁( second ▁piece ). ▁Both ▁past or ales ▁use ▁chance ▁operations ▁applied ▁using ▁charts ▁of ▁layers , ▁sounds , ▁dur ations , ▁etc . ▁and ▁the ▁I ▁Ch ing , ▁similarly ▁to ▁Music ▁of ▁Ch anges . ▁Because ▁the ▁number ▁of ▁layers ▁was ▁smaller ▁than ▁in ▁the ▁latter ▁work ▁( specific ally , ▁there ▁are ▁just ▁two ▁layers ), ▁the ▁texture ▁of ▁Two ▁Past or ales ▁is ▁th inner , ▁with ▁long ▁sil ences ▁between ▁notes . ▁Like ▁in ▁Music ▁of ▁Ch anges , ▁the ▁notation ▁is ▁proportional , ▁one ▁in ch ▁equals ▁one ▁quarter ▁note . ▁Ext ended ▁techniques ▁are ▁used : ▁strings