Romans arches were used to form bridges, aqueducts, and buildings, including the Coliseum. Arches accommodate uniform loads quite well, but concentrated load, especially at the center point, cause instability and possible collapse . Instability was solved by building wall-type extension, called spandrels, outside of arches. The Romans also discovered how to make concrete to solidify the spandrels. Horizontal support at the base of arches was also essential. Stone arches were used throughout Europe until the early 1700s.