From wikipedia:en.wikipedia.org/wiki/HD-DVD
There are many advantages to the fact that HD DVD discs will be the same size as current DVD discs. Backward compatibility will be available with all HD DVD players allowing consumers to only require a single player in their homes to play both HD DVD and DVD discs (also true with Blu-ray). DVD disc replication companies can continue using their current production equipment with only minor alterations when changing over to the format of HD DVD replication. Due to the structure of the single-lens optical head, both red and blue laser diodes can be used in smaller, more compact HD DVD players. Additionally there is a hybrid HD DVD which contains both DVD and HD DVD versions of the same movie on a single disc, providing smoother transition for the studios in terms of publishing the movies.