increase in rotation speed and 10% reduction in gross recordable capacity, accounting for the lower capacity of a single-sided dual-layer DVD at 8.5 billion bytes, versus a double-sized, single-layer DVD at 9.4 billion (for 12 cm discs). Detail differences in formatting and file structure mean the "usable" data area capacity does not change by exactly this much, but for all intents a DVD-R DL has effectively 20/11ths the capacity of a DVD-R SL, and the same holds for +R, commercially pressed, and 8 cm discs. DVD recordable discs supporting this technology are backward compatible with some existing DVD players and DVD-ROM drives. Many current DVD recorders support dual-layer technology, and the price is now comparable to that of single-layer drives, though the blank media remains more expensive. The recording speeds reached by dual-layer media are still well below those of single-layer media. There are two modes for dual layer orientation. With parallel track path (PTP), used on DVD-ROM, both layers start at the inside diameter (ID) and end at the outside diameter (OD) with the lead-out. With Opposite Track Path (OTP), used on DVD-Video, the lower layer starts at the ID and the upper layer starts at the OD where the first layer ends. The two layers share one lead-in and one lead-out. Only blank disks and drives that support the latter mode are currently available. Recordable DVD capacity comparison For comparison, the table below shows storage capacities of the four most common DVD recordable media, excluding DVD-RAM. (SL) stands for standard single-layer discs, while DL denotes the dual-layer variants. See articles on the formats in question for information on compatibility issues. See also DVD+R DL DVD DVD-R DVD-RW DVD-RW DL Book type MultiLevel Recording List of optical disc manufacturers References External links DVD-R9 and DVD+R9 Hardware, Technologies and Standards by The DVD Insider DVD Optical computer storage