DVD-9 discs are one-sided, double layer recordable DVDs with a capacity of 8.5GB. The numbering after DVDs can be approximated to the capacity of the discs. For example DVD-5 discs can hold 4.7GB on a one-sided, single layer disc and DVD-18 can hold 17.4GB on a dual-side, dual-layer disc.
In double layer (also referred to as dual layer) DVD-9 discs, two layers of standard DVD-5 are joined together with a transparent spacer and a thin reflector between the two. The bottom layer is read and written to in exactly the same manner as DVD-5. Reading and writing to the second layer is achieved by the laser focusing a fraction of a millimeter beyond the first recording layer.
DVD-9 discs are 12cm discs and can have the content data added through the production processes of either DVD duplication or DVD replication. The DVD-9 format is not as common as the DVD-5 format as the 4.7GB capacity of the DVD-5 discs is usually sufficient for most business uses, although many DVD films will use DVD-9 format if the film length is over 120 minutes.
DVDs have much higher storage capacities than CDs by using smaller pit sizes and narrower track pitches in the spiral groove that runs around the discs. These smaller pits and grooves require a different wavelength of laser light (650nm) to that used in CD drives (780nm) and therefore DVD discs cannot be read in standard CD drives.