Nationality law is the law in each country and in each jurisdiction within each country which defines the rights and obligations of citizenship within the jurisdiction and the manner in which citizenship is acquired as well as how citizenship may be lost. A person who is not a citizen of the country is generally regarded as a foreigner, also referred to as an alien. A person who has no recognised nationality or citizenship is regarded as stateless. By international custom, each sovereign state has the right to determine who it will recognise as its nationals and citizens. Such determinations may be made by custom, statutory law, or case law (precedent), or some combination. In some cases, the determination may be governed by public international law—for example, by treaties and the European Convention on Nationality.
Broadly speaking, nationality law is based either on jus soli or jus sanguinis, or on a combination of the two and even due to marital relations. Jus soli (Latin: the law of the soil) is the principle by which a child born within a country's territorial jurisdiction acquires that country's nationality. Jus sanguinis (Latin: the law of the blood) is the principle by which a child acquires the nationality of his or her parents. Today, most if not all countries apply a mixture of these two principles: neither granting citizenship to everyone born within the country's jurisdiction, nor denying citizenship to the children born abroad.
Many countries have in the past regarded marriage as an important status changing event in people's' lives and encouraged the special relationship that exists between spouses, sentiments which continue to be valued today. The common practice within and among states at the beginning of the 20th century was that a woman should have the nationality of her husband; i.e., upon marrying a foreigner the wife would automatically acquire the nationality of her husband, and lose her previous nationality, often with the reciprocal recognition by the other country. Even after the nationality of a married woman was no longer dependent on the nationality of her husband, legal provisions were still retained which automatically naturalised married women, and sometimes married men as well. This led to a number of problems, such as loss of the spouses' original nationality, the spouse losing the right to consular assistance (since consular assistance cannot be provided to nationals under the jurisdiction of a foreign state of which they are also nationals), and men becoming subject to military service obligations. There has been a shift towards a principle that neither marriage nor dissolution of marriage automatically affecting the nationality of either spouse, nor of a change of nationality by one spouse during marriage automatically affecting the nationality of their spouse. However, in many jurisdictions spouses can still obtain special and fast processing of applications for naturalisation.
International law generally recognizes the right of states to set their own policy concerning nationality. Nevertheless, there are a number of international treaties that are relevant to nationality law.
Article 29.3: The supremacy of the purposes and principles of the United Nations
Nothing in this Declaration may be interpreted as implying for any State, group or person any right to engage in any activity or to perform any act aimed at the destruction of any of the rights and freedoms set forth herein.