Twin-lens Reflex (TLR)
A medium-format camera with two stacked lenses, one for viewing through a waist-level ground glass and one for taking the picture.
In depth
Because the viewing lens has its own permanently fixed mirror, there is no blackout and no mirror slap, making a TLR unusually quiet and steady at slow speeds. The two lenses see slightly different views, producing parallax error that matters at close distances. The waist-level finder also shows a laterally reversed image, which slows composition down in a way many photographers came to value.
Examples in practice
A Rolleiflex held at chest height lets a portrait subject relax, because the photographer's face stays visible rather than hidden behind a camera.