"When connecting a digital monitor to a video card, most people use a DVI-D cable. Most monitors have only DVI-D connectors. You can't plug a DVI-I cable into a DVI-D monitor because the cable won't fit in the connector. Most video cards have DVI-I connectors because they support both analog and digital video. But some video cards have DVI-D connectors which means you can't use a DVI-I cable. I suspect that most DVI-I cables are purchased by mistake. People are hooking a DVI monitor to the DVI connector on the video card and assume that a DVI-I cable will work because it supports both analog and digital. It sounds like a universal cable. Then they discover that they can't use a DVI-I cable because either the monitor or video card has a DVI-D connector. The easiest way to deal with the issue is to use only DVI-D cables and avoid DVI-I cables. That's the standard solution: use VGA cables for connecting VGA monitors and use DVI-D cables for connecting digital monitors."

From here.