The names for electoral districts vary across countries and, occasionally, for the office being elected. The term constituency is commonly used to refer to an electoral district, especially in British English, but it can also refer to the body of eligible voters or all the residents of the represented area or only those who voted for a certain candidate.
The concept of magnitude explains Duverger's observation that Plurality voting tends to produce two-party systems, and PR methods tend to produce multi-party systems.
District magnitude is larger than 1 where multiple members are elected such as single transferable vote elections. They normally range from 2 to 10 members in a district.
In many cases, however, multi-member constituencies correspond to already existing jurisdictions (region, district, ward), which creates differences in district magnitude:
The concept of district magnitude helps explains why Duverger's speculated correlation between Proportional Representation and party system fragmentation has many counter-examples, as PR methods combined with small-sized multi-member constituencies may produce a low effective number of parties.
Larger district magnitudes annihilate the need and practice of gerrymandering, which is the practice of partisan redistricting by means of creating imbalances in the make-up of the district map. A higher magnitude means less wasted votes, and less room for such maneuvers.
Multiple-member contests sometimes use plurality block voting, which allows a single group to take all the district seats. Single voting such as SNTV or STV prevents such a landslide.
With lower district magnitudes, the only way to include demographic minorities scattered across the country is to force parties to include them:
In some places, geographical area is allowed to affect apportionment, with rural areas with sparse populations allocated more seats per elector: for example in Iceland, the Falkland Islands, Scottish islands, and (partly) in US Senate elections.
While much more difficult, gerrymandering can also be done under proportional-voting systems when districts elect very few seats. By making three-member districts in regions where a particular group has a slight majority, for instance, gerrymandering politicians can obtain 2/3 of that district's seats. Similarly, by making four-member districts in regions where the same group has slightly less than a majority, gerrymandering politicians can still secure exactly half of the seats.
However, any possible gerrymandering that theoretically could occur would be much less effective because minority groups can still elect at least one representative if they make up a significant percentage of the population (e.g. 20-25%), compared to single-member districts where 40-49% of the voters can be essentially shut out from any representation
Elected representatives may spend much of the time serving the needs or demands of individual constituents, meaning either voters or residents of their district. This is more common in assemblies with many single-member or small districts than those with fewer, larger districts. In a looser sense, corporations and other such organizations can be referred to as constituents, if they have a significant presence in an area.
In some elected assemblies, some or all constituencies may group voters based on some criterion other than, or in addition to, the location they live. Examples include: