 The space between the ribs was filled with thin panels of small pieces of stone, which were much lighter than earlier groin vaults.The outward thrust against the walls was countered by the weight of buttresses and later flying buttresses.As a result, the massive thick walls of Romanesque buildings were no longer needed; Since the vaults were supported by the columns and pillars, the walls between the columns could be thinner and higher, and filled with windows.