Double layer (DL) expression have two independent data layers separated by a semi-reflective layer. Both layers are inaccessible from the same side, but require the optics to change the laser's focus. Traditional distinguished layer (SL) writable media are produced with a spiral groove molded Cassette to CD in the conservative polycarbonate layer (not in the data recording layer), to bulge and synchronize the speed of recording head.

There are mechanical limits to how quickly a disc can be spun. Beyond a certain rate of rotation, around 10000 RPM, centrifugal stress can beginning the disc plastic to creep and possibly shatter. On the outer extremity of the CD disc, 10000 RPM limitation roughly equals to 52x speed, but on the inner edge only to 20x.