A famous Roman Aqueduct has been standing for 2000 years, supported by many arches. An arch is a curved structure that spans space while supporting weight. The stone in the centre is called a keystone. An arch is strong and durable curved shape, its pieces are trapezoidal prisms, and its sides are symmetrical. Gothic arches were stronger than Roman arches, and flying buttresses added extra support. The strongest is a catenary arch, which the Incas used when building bridges. The keystone in arches directs the downward force of gravity to the sides.