Counter-arch

Historically, the term counter-arch was used in architecture to describe multiple types of arches that provide opposing action:
 * an inverted arch used opposite a regular one. For example, an inverted arch in an open spandrel or in "Moseley bridges", a popular American Civil War-era design by Thomas William Moseley, where the counter-arches were intended as a low-cost alternative to diagonal bracing;
 * any relieving arch;
 * outer "rings" of arches overlaying the one forming the intrados, used in old English bridges since medieval times, are called "counter-arches" following the works of John Smeaton;
 * an arch that is built adjacent to another arch to oppose its forces or help stabilize it. The counter-arch can be used, for example, when constructing the flying buttress,
 * buttressing arches built between the opposing building facades over narrow streets of old cities;
 * in fortification, an arch built on the tops of counterforts behind the bastion walls intended to limit the scope of the potential wall breaching;
 * when a pier of the Old Westminster Bridge started sinking during the construction, Charles Labelye was forced to retrofit the bridge with open spandrels using the counter-arches springing off haunches of the two adjacent arches of the bridge thus relieving the pier.