"Another benefit, albeit very infrequently utilized, of the DVI specification is the ability to support both analog and digital connections on a single interface. The DVI connector can be seen below:
On the left you'll notice 3 rows of 8 pins each; these 24 pins are the only pins required to transmit the three digital channels and one clock signal. The crosshair arrangement on the right is actually a total of 5 pins that can transmit an analog video signal.
This is where the specification divides itself in two; the DVI-D connector features only the 24-pins necessary for purely digital operation while a DVI-I connector features both the 24 digital pins and the 5 analog pins...