The principle of Jus Sanguinis is older than the principle of Jus Soli. It was incorporated into the Roman Law. The principle of Jus Soli appeared at a later stage in response to the feudal theory of territorial sovereignty.
Majority of the States follow the principle of Jus Sanguinis as the basis of natural citizenship. There are, however, some States, for instance, India, Britain and the United States of America, which follow both the principles of Jus Sanguinis and Jus Soli. In these countries children born of citizen-parents become natural citizens whether born within the State or abroad.
If born within the State, they are governed by the principle of Jus Soli, and if born in some other State the principle of Jus Sanguinis is the basis to determine their natural citizenship. Since there is no uniform principle followed by the States in determining natural citizenship, very often children acquire double citizenship at the same time.
For instance, a child born of Indian parents on a visit to Britain acquires the natural citizenship of the country in which he is born in accordance with the principle of Jus Soli as well as that of India according to the principle of Jus Sanguinis.
But on attaining the age of majority, he is free to retain the citizenship of the State of his choice and relinquish that of the other. One cannot be a citizen of two States at the same time.