The DualDisc is a CD/DVD hybrid. One side may be used as an audio CD. The other side can be used as a DVD. In addition, it may contain extras such as photos, lyrics, hyperlinks, advertising or music with more than two channels. However, because the CD side of a DualDisc is thinner than prescribed in the Red Book standard for audio CDs (0.9 mm instead of the 1.1 mm minimum), it may not be playable in all CD players. It is therefore advisable to check the manual or product description of your CD player before purchasing a DualDisc.