"Universal suffrage means in principle that all human beings have the right to vote. This right may, however, and indeed should, be subject to certain conditions: c. Residence: i. a residence requirement may be imposed; ii. residence in this case means habitual residence; iii. a length of residence requirement may be imposed on nationals solely for local or regional elections; iv. the requisite period of residence should be reasonable and, as a rule, should not exceed six months; v. it is desirable that the right to vote be accorded to citizens residing abroad. "
CoE (Venice Commission): Code of Good Practice on Referendums, para. 1.1.c

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Code of Good Practice on Referendums