DVD±9 format, Double Layer recording technology, also known as Dual Layer DVD, allows for more data to be stored on a single DVD disc – up to 8.55 GB in total-disc capacity, almost double that of 4.7 GB single-disc capacity. Since DVD±9 inception, double layer use has increased steadily over the years, especially as prices dropped.

Sidenote: Which format should I use + or -? With Double Layer blank media, DVD+R DL is by far the most used format. With Single Layer blank media, DVD-R is the most popular format.

So how does Double Layer recording technology work?

In simple terms, dual layer discs are formed with a second physical layer within the disc. The DVD disc reader or player is able to access the second layer of data by shining a laser through the first, transparent layer.

In more technical terms, dual layer discs are formed with two organic dye-recording layers (L0 and L1), which are separated by a single transparent spacing layer. Unlike single layer recordings, with dual layer discs each recording layer has its own wobble pre-groove that’s molded into the polycarbonate base, allowing for individualized control of the disc rotation speed and addressing scheme.  The table of contents in dual layer recording is always embedded in the L0, the first recordable layer.

Two modes exist for dual-layer orientation. The parallel track path (PTP) is used for DVD-ROM. Data is laid down in PTP mode from beginning of the first layer to the end of the first layer and then from the beginning of the second layer to the end. Sequencing for the other dual-layer orientation, Opposite Track Path (OTP), begins with the first layer to the end of the first layer, but then moves from the end of the second layer to the beginning of the second layer.

*Source: Burnworld.com

Disc Capacity Comparison

For comparison, the table below shows storage capacities of the four most common DVD recordable media, excluding DVD-RAM. (SL) stands for standard single-layer discs, while DL denotes the dual-layer variants.

*Chart source: Wikipedia