DVD+R is a writable optical disc with a 4.7GB capacity developed by a group of corporations known as the DVD+RW Alliance, and is a competing format to the DVD-R, developed by the DVD Forum. The process for manufacturing DVD-R and DVD+R discs is virtually identical. The main difference between the two formats is the manner in which the location of the laser beam on the disc is determined. DVD-R discs use prepits along the grooves of the discs to determine laser position, while DVD+R measures the wobble frequency of the laser moving toward the outer edge of the disc. Older DVD burners often only accepted one format or the other, but most current burners are hybrid, allowing the use of either type. DVD+RW are practically identical to DVD+R except that DVD+RW can be written to repeatedly, where as DVD+R can only be written once.