RSDL (Reverse Spiral Dual Layer) DVDs are the most common form of dual layer discs. Layer one of a RSDL disc is read from the inside to the outside, while layer two is read from the outside to the inside. This results in a minimal amount of time for transition between layers. Some DVD-9 and DVD-18 discs are not RSDL and when changin layers, the DVD laser must return to the inside track of the disc before commencing playback on the second layer. This results in a much lengthier pause between layers.