Lithuanian nationality law operates on the jus sanguinus principle, whereby persons who have a claim to Lithuanian ancestry, either through parents, grandparents, great-grandparents may claim Lithuanian nationality. Citizenship may also be granted by naturalization. Naturalization requires a residency period, an examination in the Lithuanian language, examination results demonstrating familiarity with the Lithuanian Constitution, a demonstrated means of support, and an oath of loyalty. A right of return clause was included in the 1991 constitution for persons who left Lithuania after its occupation by the Soviet Union in 1940 and their descendants. Lithuanian citizens are also citizens of the European Union and thus enjoy rights of free movement and have the right to vote in elections for the European Parliament.