known as reciprocal licenses: any modifiers of a copyleft-licensed work are expected to reciprocate the author's action of copyleft-licensing the software by also copyleft-licensing any derivatives they might have made. Because of this requirement, copyleft licenses have also been described as "viral" due to their self-perpetuating terms.

In addition to restrictions on copying, copyleft licenses address other possible impediments. They ensure that rights cannot be later revoked,