A DVD-18 is a double sided, dual layered DVD. It can hold 17 gigabytes of data, which comes down to approximately 482 minutes of video and audio.
The discs are 12 cm (4.7 inches) wide, and 1.2 mm thick.
A DVD-18 is basically two DVD-9s in one disc. This means that there are four layers on the disc, but the data is divided over two sides, so the disc needs to be flipped. Usually, the movie is on one side, and most of the extras on the other. A fine example of a DVD-18 is the Terminator 2: Ultimate Edition DVD. Unfortunately, the technique to make a DVD-18 is very expensive, and therefor usually 2 DVD-9s are used instead of a DVD-18. An example of this is the Fight Club DVD. | 
