An arch is a pure compression form.[4][5][6][7] It can span a large area by resolving forces into compressive stresses, and thereby eliminating tensile stresses. This is sometimes denominated "arch action".[8] As the forces in the arch are transferred to its base, the arch pushes outward at its base, denominated "thrust". As the rise, i. e. height, of the arch decreases the outward thrust increases.[9] In order to preserve arch action and prevent collapse of the arch, the thrust must be restrained, either by internal ties or external bracing, such as abutments.[10]