The leader or chairman of a parliamentary group holds an influential political post in a parliamentary system with strong party discipline

Party discipline

Party discipline is the ability of a parliamentary group of a political party to get its members to support the policies of their party leadership. In liberal democracies, it usually refers to the control that party leaders have over its legislature...

. When a party is in opposition

Opposition (parliamentary)

Parliamentary opposition is a form of political opposition to a designated government, particularly in a Westminster-based parliamentary system. Note that this article uses the term government as it is used in Parliamentary systems, i.e. meaning the administration or the cabinet rather than the state...

, or the minor or junior partner in a governing coalition

Coalition government

A coalition government is a cabinet of a parliamentary government in which several political parties cooperate. The usual reason given for this arrangement is that no party on its own can achieve a majority in the parliament...

, they are often the political leader of the party and the primary spokesperson for the party, although this is also is also true of the governing party in some cases. When a parliamentary group gains the confidence of the legislature

Confidence and supply

In a parliamentary democracy confidence and supply are required for a government to hold power. A confidence and supply agreement is an agreement that a minor party or independent member of parliament will support the government in motions of confidence and appropriation votes by voting in favour...

 (that is, secures the most seats in the legislature), the leader of that group often becomes the Prime Minister

Prime minister

A prime minister is the most senior minister of cabinet in the executive branch of government in a parliamentary system. In many systems, the prime minister selects and may dismiss other members of the cabinet, and allocates posts to members within the government. In most systems, the prime...

 or Premier

Premier

Premier is a title for the head of government in some countries and states.-Examples by country:In many nations, "premier" is used interchangeably with "prime minister"...

.