A rangefinder camera has a viewfinder window built into its front and a second rangefinder window off to its side. This optical system, separate from the imaging lens, is what you use to focus the camera. This is what differentiates a rangefinder camera from a ‘single lens reflex’ camera, which uses the imaging lens itself as the optical system for viewing the scene to be photographed.
The rangefinder camera’s viewfinder window generates the view you see when looking through the viewfinder itself. To the right of the viewfinder window (when using the camera) is located a second smaller window, the rangefinder window, which itself sits in front of a moveable mirror that reflects a second image to the viewfinder. This mirror moves as the lens focus ring is adjusted.The reflection from the mirror passes to a small lens before reaching a half-silvered, beam splitter mirror located in the main viewfinder.
This reflected second image is referred to as the rangefinder patch. It is projected into the center portion of the viewfinder image. The twin images of the subject in the viewfinder are superimposed via the focus ring of the lens. On a coupled-rangefinder the focus ring moves a small sensor arm in the camera body that pivots the movable mirror as the focus is set. When you adjust the focus ring on the lens the small image projected from the the rangefinder window will appear to shift sideways in relation to main viewfinder image. Once you see these two images coincide to form a single clear image, your lens is properly focused.