What is Porro Prism or Roof Prism system?

Since the objective lenses form images that are both upside down and reversed left for right, prisms are used to invert the primary image.

Porro prism and roof prism are two most frequently used systems for such a purpose. Porro prism binoculars are characterised by the eyepieces being offset from the objective lenses, while roof prism binoculars' objective lens and eyepiece are positioned in line for a more compact feature and reduced weight. Either type of prism system, properly manufactured, gives excellent optical performance.