The apple hdmi to dvi adapter allows you to connect your mac with built in hdmi to a dvi display or projector you can use an external display as your primary display extend your desktop to a second display or work in mirroring mode with a projector so you can view what your audience sees use the hdmi to dvi adapter to connect a single link dvi digital monitor such as the 20 inch or 23 inch apple cinema display that includes a dvi connector