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The history of Porro Prisms dates back to 1854 when Ignazio Porro invented it. It is a type of reflection prism consisting of two prisms in an optical instrument to invert and reverse the orientation of an image by 180 degrees. Porro prisms are used in many optical viewing instruments, such as periscopes, binoculars, and monoculars. They are most often used in pairs, forming a double Porro prism. Double Porro prism systems are used in small optical telescopes to re-orient an inverted image. In many binoculars they both erect the image and provide a longer folded distance between the objective lenses and the eyepieces. The major difference between a roof prism and a Porro prism is that for the roof prism the roof edge is coplanar with the entrance and exit beam, while for a Porro prism the roof edge is orthogonal to the plane formed by the beams.