The spreader is a standing rigging spar in the form of a horizontal bar perpendicular to the mast and oriented athwartship. It may be slightly angled toward bow or stern.
The primary role of the spreader is to guide the shrouds supporting the mast so that the mast will assume the proper shape under load. Its elevated position allows it to be used to mount radar reflectors, lights, antennas, etc.
The spreader is a bar, usually of metal, mounted athwartship on the mast. It is usually made of the same material as the mast. The number of spreaders will depend on the height and design of the mast.
Choice will depend on the material, the design and, above all, the size of the spreader.