▁econom ics , ▁history , ▁ge ography ▁and ▁soci ology . ▁ ▁Mon uments ▁are ▁generally ▁located ▁in ▁par ks ▁and ▁gard ens , ▁squares ▁and ▁inter sections , ▁or ▁near ▁public ▁buildings . ▁There ▁are ▁several ▁types ; ▁until ▁the ▁ 1 9 th ▁century ▁the ▁most ▁common ▁were ▁the ▁column , ▁the ▁tri um phal ▁arch ▁and ▁the ▁e quest rian ▁statue , ▁three ▁types ▁of ▁monuments ▁inherited ▁from ▁Roman ▁art . ▁Later ▁monuments ▁were ▁b ust s ▁placed ▁in ▁nich es , ▁med all ions , ▁f ount ains , ▁st eles ▁and ▁ped est als , ▁fr ie zes , ▁relief s , ▁comm emor ative ▁pla ques , ▁grav est ones , ▁followed ▁by ▁install ations , ▁land ▁art ▁and ▁mult id is cipl inary ▁works . ▁Comm only - used ▁materials ▁include ▁wood , ▁pl aster , ▁ter rac otta ▁and ▁other ▁cer am ics , ▁stone , ▁mar ble , ▁bronze , ▁iron , ▁steel , ▁concrete ▁and ▁al umin ium . ▁ ▁Th emes ▁include ▁hom age ▁to ▁a ▁character ▁or ▁event : ▁religious , ▁myth ological , ▁symbol ic , ▁alleg or ical , ▁historical , ▁political , ▁military , ▁scientific ▁or ▁art istic . ▁Public ▁art works ▁comm emor ate ▁many ▁aspects ▁of ▁the ▁history ▁of ▁Barcelona . ▁ ▁History ▁ ▁Middle ▁A ges ▁ ▁Barcelona ' s ▁first ▁public ▁art works ▁were ▁created ▁during ▁the ▁Middle ▁A ges , ▁when ▁the ▁city ▁formed ▁part ▁of ▁the ▁Crown ▁of ▁Arag on ▁and ▁was