"As regards, in particular, the choice of electoral system, the Court reiterates that the Contracting States enjoy a wide margin of appreciation in this sphere. In that regard, Article 3 of Protocol No. 1 goes no further than prescribing “free” elections held at “reasonable intervals”, “by secret ballot” and “under conditions which will ensure the free expression of the opinion of the people”. Subject to that reservation, it does not create any “obligation to introduce a specific system” such as proportional representation or majority voting with one or two ballots (see Mathieu-Mohin and Clerfayt, cited above, § 54). "
CoE (ECHR): Case of Sitaropoulos and Giakoumopoulos v. Greece, para. 65

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Case of Sitaropoulos and Giakoumopoulos v. Greece