A grand coalition is an arrangement in a multi-party parliamentary system in which the two largest political parties of opposing political ideologies unite in a coalition government. The term is most commonly used in countries where there are two dominant parties with different ideological orientations, and a number of smaller parties large enough to secure representation in the parliament. The two large parties will each try to secure enough seats in any election to have a majority government alone, and if this fails each will attempt to form a coalition with smaller parties that have a similar ideological orientation. Because the two large parties will tend to differ on major ideological issues, and portray themselves as rivals, or even sometimes enemies, they will usually find it more difficult to agree on a common direction for a combined government with each other than with smaller parties.
Causes of a grand coalition 
Occasionally circumstances arise where normally opposing parties may find it desirable to form a government. One is a national crisis such as a war or depression, where people feel a need for national unity and stability that overcomes ordinary ideological differences. This is especially true where there is broad agreement about the best policy to deal with the crisis. In this case, a grand coalition may occur even when one party has enough seats to govern alone. An example would be the United Kingdom national governments during World War I and before and during World War II.
Another possibility is that the major parties may find they have more in common ideologically with each other than with the smaller parties, or that the fragmentation of the smaller parties is so great that no other coalition is stable. Examples include Austria, where the mainstream parties of the left and right have often formed grand coalitions to keep parties of the far left or far right out of government (an example of a cordon sanitaire), or Israel, where in some parliaments the fragmentation and intransigence of some of the smaller parties has made it easier to maintain a coherent platform with a grand coalition than with a narrow one. This is often done out of political necessity, to prevent an early election.
See also 
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