Toshiba and Memory-Tech show a single-sided, three-layer ROM disc supporting storage and playback of both DVD and HD DVD formats.

Compared to the current TWIN format disc the new three-layer offers 30GB HD DVD capacity (current TWIN format disc only offers 15GB), plus 4.7GB of standard DVD capacity.
The new disc can also be configured to maximize standard DVD storage to 8.7GB and go with 15GB on the HD DVD side.
Memory-Tech and Toshiba together proposed the DVD/HD DVD TWIN to the DVD Forum in 2004, as a transitional format able to support both standard DVD and HD DVD content.
More details in this Toshiba press-release.