The approval of DVD specifications for high density read-only disc [HD DVD-ROM (51G)] part 1 physical specifications, version 2.0, took place during the 40th steering committee meeting on November 15, 2007. Back in September, the DVD Forum also approved version 1.9 specification of 51GB HD DVD media as well as revision 1.0 of triple-layer twin format discs, which can hold up to 30GB of data on its HD DVD side and up to 4.7GB of data on its DVD side.Unfortunately it's not clear if tri-layer HD DVDs will be readable by current HD DVD hardware.
The new 51GB HD DVD ROM disc has a three-layer structure with each layer storing 17GB of data, which is an advancement in capacity over current ROM discs, which hold 15GB of data in each layer of a single-sided disc. Continued improvement in disc mastering technology has achieved further minimization in the recording pit, supporting a further boost in capacity to 17GB in single layer and a full 51GB on a single-sided triple-layer disc. Toshiba has confirmed the disc structure and its successful operation earlier this year.
Source: X-bit Labs