One type of binocular. These are larger and heavier than their counterpart (roof prisms), and often feature a larger field of view.
Binocular prism glass works inside the binocular to flip the image upright for viewing. The two most common prism designs are porro and roof prisms. Porro prisms work in a simple zig-zag fashion to flip the image and account for the classic binocular shape. This design makes porros simple to manufacture and less expensive overall. However, the porro design also adds more bulk and weight to the binocular.
Most common type of prism (“triangular”) which inverts image and reverses the direction of the light.