An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments on either side. A viaduct (a long bridge) may be made from a series of arches.
An arch bridge works by conveying the downward pressure of gravity inward to the center of the structure — toward a central stone called the key part — rather than straight down. This principle is called compression, and it enables the arch below to support the surface, or deck, above it.
(Fixed arch bridges can be destabilized by temperature fluctuations, so the arch design is sometimes modified with hinges at each base and even the center of the span. This helps longer arch bridges adapt to the expansion or contraction of their materials when temperatures change drastically.)