Dual format disc
 

Memory-Tech Corporation and Toshiba have jointly developed a dual-layer ROM (read-only memory) disc that can store content in both the HD DVD and DVD formats.

The newly developed ROM disc has a single-sided, dual-layer structure. The upper layer, closer to the optical head, stores data in the DVD format, and the lower layer stores HD DVD data. The DVD layer has a 4.7GByte capacity, satisfying specification of current DVD discs, while the HD DVD layer can store 15GBytes.

Read more here