Below we examine the different types of UK electoral systems and how they affect political party systems.
First Past the Post helps to entrench the two party system and makes it difficult for new parties to get represented.
A party may get 15-20% across the country and get only a handful of seats, as the Liberals did in the 1970s.
However, if a party has local pockets of strong support as Plaid Cymru does in Welsh speaking Wales, the SNP have in N.E Scotland and the Liberals did in the Scottish Highlands and S.W. England in the 1960s and 1970s it can win seats. So, FPTP may not necessarily prevent the development of a multi-party system.
The growth of tactical voting with Labour and Conservative supporters voting for the Liberal Democrats to keep the other party out has increased and in fact at the 2015 General Election some newspapers were telling voters who to vote for tactically where they lived to get someone out or someone in.
The Alternative Vote is also a majoritarian system and requires a candidate to gain 5O% of the vote to win.
It is still the two main parties that benefit as they will be in the top two places in most constituencies in first preference votes.
However, the system also entrenches tactical voting as so helps the Liberal Democrats were they are stronger and strengthens the two and a half party system.
It is likely to strengthen the most popular of the two parties as the voters choose it for their other preference votes and even push the system towards a dominant party one, if only temporarily.
Proportional Representation almost inevitably creates a multi-party system although the number of parties gaining representation in Parliament depends on whether a threshold is required so that a party needs a minimum percentage of votes to get any MPs at all.
It also depends on the ideological composition of the electorate so that the more different types of outlook there are then the more parties can gain support.
The system will not normally produce big shifts in the number of seats that the parties win from one election to the next.
The Single Transferable Vote system of PR will produce a less fragmented party system than the party list system, as candidates have to reach the threshold by gaining lower preference votes.
It also makes it easier for Independent candidates to enter Parliament as voters of different parties may give them a preference vote.
The Additional Member System, which has elements of both FPTP and PR, will also make it difficult for very small parties to enter Parliament but will give more representation to parties other than the largest two.
