1. It is a variant of the more common double Porro prism configuration. 2. Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms . 3. Porro prism binoculars do not split beams and therefore they do not require any phase coatings.4. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation.5. Porro prisms are most often used in pairs, forming a "'double Porro prism " '.6. Porro prisms are most often used in pairs, forming a "'double Porro prism " '. 7. Galilean designs were widely used up to the end of the 19th century when they gave way to porro prism types. 8. An image travelling through a Porro prism is rotated by 180?and exits in the opposite direction offset from its entrance point. 9. Therefore, to reduce the cost of production for a porro prism , the edge of the roof is usually left out. 10. While a single porro prism can be constructed to work as well as a roof prism, it is seldom used as such.