▁center ▁is ▁usually ▁not ▁considered ▁to ▁form ▁a ▁tw in ▁city ▁with ▁it . ▁For ▁example , ▁South ▁San ▁Francisco ▁( pop ulation ▁about ▁ 6 5 , 0 0 0 ) ▁is ▁not ▁considered ▁a ▁tw in ▁city ▁with ▁San ▁Francisco ▁( pop ulation ▁about ▁ 8 5 0 , 0 0 0 ). ▁However , ▁cities ▁considered ▁tw inned ▁by ▁proxim ity ▁do ▁not ▁necessarily ▁match ▁dem ograph ically , ▁econom ically , ▁or ▁polit ically . ▁ ▁In ▁many ▁historical ▁cases , ▁cities ▁that ▁grew ▁into ▁each ▁other ' s ▁space ▁lost ▁their ▁individual ▁ident ities , ▁and ▁the ▁border ▁or ▁bar rier ▁originally ▁separ ating ▁them ▁became ▁almost ▁irrelevant . ▁An ▁ 1 8 7 3 ▁case ▁of ▁tw in ▁cities ▁mer ging ▁to ▁become ▁a ▁un ited ▁city ▁is ▁Budapest ▁in ▁Hung ary , ▁which ▁began ▁as ▁two ▁settlement s ▁( B uda ▁and ▁P est ) ▁facing ▁each ▁other ▁across ▁the ▁Dan ube ▁at ▁a ▁strateg ic ▁for ding ▁place ▁along ▁a ▁trade ▁route . ▁ ▁In ▁China , ▁the ▁three ▁ancient ▁cities ▁of ▁H ank ou , ▁H any ang , ▁and ▁W uch ang , ▁separated ▁by ▁the ▁j unction ▁of ▁the ▁Yang t ze ▁and ▁Han ji ang ▁rivers , ▁were ▁joined ▁in ▁ 1 9 2 7 ▁into ▁the ▁single ▁entity ▁of ▁Wu han . ▁ ▁T win ▁cities ▁may ▁share ▁an ▁air port ▁into ▁whose ▁air port ▁codes ▁are ▁integrated ▁the ▁component ▁initial s , ▁e . g ., ▁D F W