From GEAR Software Knowledgebase
DVD-5 have one information surface on one side. DVD-10 discs have one information surface per side. DVD-9 discs have 2 information surfaces on one side, separated by a precise amount (55mm ± 15 mm; 55mm ± 10 mm variation within one disc; ± 4 mm variation within one revolution), formed by the bonding agent. These information planes are called layers (the first layer is layer 0, layer 1 is behind layer 0). Layer 0 must be partially reflective so that the read laser can be focused through layer 0 to layer 1, which is fully reflective.