DVD-R DL

DVD-R DL (DL stands for Dual Layer), also called DVD-R9, is a derivative of the DVD-R format standard. DVD-R DL discs employ two recordable dye layers, each capable of storing nearly the 4.7 GB (4.38 GiB) of a single-layer disc, almost doubling the total disc capacity to 8.54 GB (7.96 GiB). Discs can be read in many DVD devices (older units are less compatible) and can only be written using DVD-R DL compatible recorders.
See more at Wikipedia.org...

DVD+R DL (DL stands for Dual Layer, commonly mistaken as Double Layer), also called DVD+R9, is a derivative of the DVD+R format created by the DVD+RW Alliance. Its use was first demonstrated in October 2003. DVD+R DL discs employ two recordable dye layers, each capable of storing nearly the 4.7 GB capacity of a single-layer disc, almost doubling the total disc capacity to 8.55 GB (or 7.96 GiB). Discs can be read in many DVD devices (older units are less compatible) and can only be created using DVD+R DL and Super Multi drives. DL drives started appearing on the market during mid 2004, at prices comparable to those of existing single-layer drives. However, the price of DL media ($1 to $5 USD per disc) is many times that of single-layer media. The latest DL drives write dual layer discs at a slower rate (up to 10×) than current single-layer media (up to 20×).
See more at Wikipedia.org...