Camera Cage
A rigid frame that surrounds a camera body, providing mounting points for monitors, microphones, handles and rigging.
In depth
A stills body has one tripod thread, which is nowhere near enough for a video build. A cage distributes the load across the body's structure and adds threaded holes on every face, along with cable clamps that prevent an HDMI port being torn out. It also protects the body in a way the magnesium shell alone does not.
Examples in practice
A cage carries a monitor on top, a microphone on a cold shoe and a side handle, none of which the bare body could support.