The purpose of a prism is to show an upright image — without a prism, you'd see a reversed, upside-down image, so the prism's job is to correct that. There are two types of prism: Roof and Porro. Porro prism binoculars tend to be the larger of the two to accommodate the internal layout of the prisms (the binoculars are often described as being 'broad-shouldered', where the eyepieces don't line up with the objective lenses). Due to this internal layout, each turn of light throughout the optical path is 90 degrees to avoid dispersing the light rays, resulting in a brighter, true-to-color three-dimensional image. Porro binoculars have an external focus wheel, whereas roof prism binos feature an internal focus worm. Roof prisms are considerably more compact, and typically more popular than porro prism designs — to help you tell them apart, porro prism binoculars look like the letter M, and roof prism binos look like the letter H. Their light path looks more triangular, like the roof of a house, hence the name. Roof prism binoculars are also easier to seal, making for better waterproofing qualities.