Although DVDs (Digital Versatile/Video Discs) all look the same, there are several different types. One of them is DVD9, which offers a big upgrade over a basic DVD disc and can hold many more features.
DVD9 refers to a version of the DVD media format that holds 8.5GB of information. Other names for DVD9 include Single Sided Dual Layered, DVD-R9, DVD-R DL and DVD+R9.
A DVD9 disc cannot hold or write the complete 8.5GB that manufacturers claim (nor can any other storage medium). Computers deal in packets of information with a size of 1024 bytes, but storage media only use 1000-byte clusters. After this conversion factor, a DVD9 disc only holds 7.95GB.
DVD9 holds almost twice as much information as the DVD5 format (which holds 4.38GB in practice), because DVD9 contains two layers of information, known as dual or double layer.
A lot of films and entertainment media released on the DVD format use DVD9 because it offers more space for extras such as trailers, deleted scenes and uses less compression, leading to a higher-quality picture.
Only a DVD+R burner can write to a blank DVD9 formatted disc. Most burning software can write to a DVD9 disc, but during playback, the DVD player stops for a brief moment to read off of the second layer. However, people rarely notice this "skip."