the United States. If an individual with Czech ancestry has ascendants who naturalized in a country other than the United States and is thus unable to prove loss of citizenship by naturalization due to a lack of a treaty between their country and Czechoslovakia that restricted dual citizenship, they may be eligible for Czech citizenship by proving a loss of an ancestor's citizenship by other means. One such way could be if their ancestor was a grandmother who married a non-Czechoslovak citizen before June 24th, 1947, in which case their grandmother would have automatically lost Czechoslovak citizenship ipso iure due to the Emigrant Patent of Austrian Law from 1832. The proof of loss of citizenship in this case would be the grandmother's marriage certificate. Loss of citizenship Voluntary (restrictions on dual citizenship) Czech citizenship can be renounced voluntarily if doing so wouldn't cause one to be stateless unless it is in connection with a marriage or by birth – the Czech Republic does not require children born with another nationality to renounce it upon reaching maturity. Involuntary The involuntary loss of citizenship is constitutionally prohibited. However, before the 2013 Citizenship Act (effective as of 1 January 2014) it was sometimes argued by emigrants and emigrant groups that the restrictions on dual citizenship were a form of involuntary deprivation of citizenship. During the communist era (1948–89) hundreds of thousands of Czechoslovakian citizens had emigrated into other parts of the West. The regime punished emigration by removing Czechoslovak citizenship, along with property confiscation and in absentia prison sentences. Since the Velvet Revolution in 1989, many emigrants demanded their citizenship be restored. Between 1999 and 2004, a special measure allowed them to regain the citizenship, but a few people took advantage of the wording, which "granted" citizenship rather than "restored" it and so got dual citizenship. A few people from Volhynia and Romania also got citizenship. Proof of citizenship Czech citizenship can be proved by A national identification card (občanský průkaz) (mandatory for those over 15 years of age), A travel document; e.g. a Czech passport, A certificate of Czech citizenship (CCC) issued by the Ministry of the Interior that is less than 12 months old since the date of issuance, A Czech marriage certificate, but only if the citizenship details are included. Dual citizenship Czech Republic allows its citizens to hold foreign citizenship in addition to their Czech citizenship since 2014. The