Two-party system
Third parties, meaning a party other than one of the two dominant parties, are possible in two-party systems, but they are often unlikely to exert much influence by gaining control of legislatures or by winning elections. While there are occasional opinions in the media expressed about the possibility of third parties emerging in the United States, for example, political insiders such as the 1980 presidential candidate John Anderson think the chances of one appearing in the early twenty-first century is remote. A report in "The Guardian" suggested that American politics has been "stuck in a two-way fight between Republicans and Democrats" since the Civil War, and that third-party runs had little meaningful success.
Political party
The UK political system, while technically a multi-party system, has functioned generally as a two-party (sometimes called a "two-and-a-half party") system; since the 1920s the two largest political parties have been the Conservative Party and the Labour Party. Before the Labour Party rose in British politics the Liberal Party was the other major political party along with the Conservatives. Though coalition and minority governments have been an occasional feature of parliamentary politics, the first-past-the-post electoral system used for general elections tends to maintain the dominance of these two parties, though each has in the past century relied upon a third party to deliver a working majority in Parliament. (A plurality voting system usually leads to a two-party system, a relationship described by Maurice Duverger and known as Duverger's Law.) There are also numerous other parties that hold or have held a number of seats in Parliament.