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DVD-9 is a DVD with two layers. These DVDs can hold roughly twice the data, approximately 8.75 gigabytes compared to 4.7 of a standard DVD. The term refers to both commercially produced DVDs and writable DVDs.
Because these discs have two layers, they are also known as dual-layer DVDs.
A DVD-9 uses two different layers on one side of the disc to double the amount of data it can store. There is a semitransparent spacer between the two layers of the disc, typically made out of gold. This can easily be seen with commercially-produced dual-layer DVDs on the underside of the disc. Many Hollywood movies on DVD use dual-layer discs because the extra capacity lets studios release DVDs with better picture quality while enabling special features like commentary tracks.
The second layer starts at the edge of the disc and moves inward, while the first layer starts on the inside and moves outward. When viewing DVD movies, there may be a momentary pause in the middle as the DVD player’s laser changes layers. Some studios have put a disclaimer on the DVD packaging explaining that this is normal and not an indicator that a disc is damaged or defective.
In addition to commercial discs, DVD-9 discs are available in writable formats. They’re sold as “DVD-R DL” and “DVD+R DL,” where “DL” stands for “dual-layer.” Where commercial discs are physically stamped, these writable discs are like CD-R and CD-RW discs, where the laser changes the color of the dye on the underside of the disc to represent the binary pattern of 0s and 1s.