The principle of equal voting power, as recognised by national constitutions and international law, implies that: seats must be evenly distributed among the constituencies. Countries with only a nationwide constituency do not use any allocation criterion. In other countries there are different ways to determine either the boundaries of the constituencies or the number of seats per constituency in case of multi-member constituencies. The Code of Good Practice in Electoral Matters mentions four possible allocation criteria: population, number of resident nationals (including minors), number of registered voters, and the number of people actually voting. An appropriate combination of these criteria might be envisaged too. While the last criterion does not appear to be applied and could be problematic, national legislation provides for the three other ones, mostly population and number of registered voters. The choice of the allocation criterion may be very relevant: the representation of constituencies with large number of foreign population (mainly urban centres) will be much higher if the population criterion rather than the number of registered voters is applied, while using the number of resident nationals will favour constituencies with a younger population.