Technology

• Dual layer DVDs contain two writable surfaces. The two layers are sealed together with a transparent material. Dual layer DVDs are the same physical size as single layer DVDs. The greater complexity required to produce dual layer DVDs means they are much more expensive than single layer DVDs.
  
  

Capacity

• Dual layer DVDs can hold twice as much data as single layer DVDs. Dual layer DVDs hold about 8.5 GB of data, whereas single layer DVDs hold only 4.7 GB of data. Because 4.7 GB is sufficient for most uses, dual layer DVDs are not as common as single layer DVDs.
  
  

Readers and Writers

• Reading the data written on a dual layer or writing new data onto a dual layer DVD requires the DVD player to adjust the depth of its laser. Although most DVD players are able to read dual layer DVDs, dual layer DVDs can only be produced by specialized DVD recorders. Dual layer DVDs can be written to, but it is not possible to re-write dual layer DVDs.