Compact disks have been a popular medium for distributing software, multimedia
data such as audio and images, and other electronically published information.
They have a storage capacity of 640 to 700 megabytes, and they are cheap
to mass-produce. Digital video disks (DVDs) have now replaced compact disks
in applications that require larger amounts of data. Disks in the DVD-5 format
can store 4.7 gigabytes of data (in one recording layer), while disks in the DVD-9
format can store 8.5 gigabytes of data (in two recording layers). Recording on
both sides of a disk yields even larger capacities; DVD-10 and DVD-18 formats,
which are the two-sided versions of DVD-5 and DVD-9, can store 9.4 gigabytes