The more fluid nature of politics in Northern Ireland makes it harder to define all major parties. In addition many by-elections have not been contested by parties holding other seats in the House of Commons, whether due to agreements with other parties, poor organisation in the constituency or the particular circumstances on the by-election. However, for the period since 1981 (which saw the first by-elections in twelve years, during which time several major political realignments had occurred) the main parties are usually considered to be the Democratic Unionist Party, Sinn Féin, the Social Democratic and Labour Party (SDLP) and the Ulster Unionist Party.