▁there ▁are ▁a ▁few ▁fragment ary ▁t unes ▁for ▁which ▁only ▁a ▁ground ▁is ▁known . ▁ ▁In ▁most ▁cases ▁the ▁variations ▁following ▁the ▁ground ▁involve ▁the ▁use ▁of ▁a ▁number ▁of ▁different ▁musical ▁emb ell ish ments , ▁usually ▁starting ▁very ▁simply ▁and ▁progress ing ▁through ▁success ively ▁more ▁complex ▁movements ▁before ▁returning ▁again ▁to ▁the ▁ground . ▁ ▁Vari ations ▁after ▁the ▁ ù rl ar ▁or ▁ground ▁usually ▁include ▁a ▁si ub hal ▁(" pass ing " ▁or ▁" tra vers ing ") ▁or ▁d ith is ▁(" two " ▁or ▁" a ▁pair ") ▁or ▁both . ▁ ▁The ▁si ub hal ▁compr ises ▁theme ▁notes ▁each ▁couple d ▁with ▁a ▁single ▁note ▁of ▁higher ▁or ▁lower ▁pitch ▁that ▁usually ▁preced es ▁the ▁theme ▁note . ▁ ▁The ▁theme ▁note ▁is ▁held ▁and ▁its ▁pa ired ▁single ▁note ▁cut . ▁The ▁timing ▁given ▁to ▁the ▁theme ▁notes ▁is ▁of ▁critical ▁importance ▁in ▁displaying ▁the ▁virt u os ity ▁of ▁the ▁master ▁pi per . ▁If ▁the ▁theme ▁and ▁single ▁note ▁are ▁repeated ▁or ▁played ▁in ▁pairs , ▁it ▁is ▁referred ▁to ▁as ▁a ▁dou bling , ▁otherwise ▁a ▁si ub hal ▁sing ling . ▁ ▁The ▁d ith is ▁is ▁similar . ▁ ▁The ▁theme ▁note ▁is ▁acc ented ▁and ▁followed ▁by ▁a ▁cut ▁note ▁of ▁lower ▁pitch , ▁usually ▁altern ating , ▁for ▁example , ▁between ▁an ▁A ▁and ▁a ▁G . ▁ ▁If ▁the ▁couple d ▁pairs ▁are ▁played ▁in ▁a ▁repeating ▁pattern , ▁it ▁too ▁is