A double majority is the name given to a vote which requires a majority of votes according to two separate criteria. The mechanism is usually used to require strong support for any measure considered to be of great importance. Typically in legislative bodies, a double majority requirement exists in the form of a quorum being necessary for legislation to be passed.
Examples of double majority 
In Australia, constitutional changes must be passed at a referendum in a majority of states (4 of the 6), and by a majority of voters nationally. Prior to 1977, the votes of citizens in the Northern Territory and the ACT did not affect the national or state-based count. After a Constitution Alteration put to referendum in 1977 and given vice-regal assent on 19 July 1977, Territorial votes contribute towards the national majority, but the Territories themselves do not count towards the majority of states. Note that the territories have very small populations.
Since the patriation of the Canadian constitution in 1982, thorough amending formulae for the constitution were adopted. Per the Constitution Act, 1982, many amendments can be passed only by the Parliament of Canada and a two-thirds majority of the provincial legislatures, those provinces together representing at least 50% of the national population-–this is known as the 7/10 formula (as there were and are 10 provinces, so 7 constitutes a two-thirds majority). Additionally, a province can explicitly choose to dissent to such an amendment, in which case it does not apply in that province even if passed. Though not constitutionally mandated, a referendum is also considered to be necessary by many, especially following the precedent established by the Charlottetown Accord in 1992.
However, there are some parts of the constitution that can be modified only by a vote of all the provinces plus the Parliament of Canada; these include changes to the composition of the Supreme Court of Canada, changing the process for amending the constitution itself, or any act affecting the Canadian monarch or governor general.
European Union 
In the European Union, double majority voting is a form of Qualified Majority Voting which is to apply to almost all policy areas starting in 2014 under the Treaty of Lisbon. Any decision taken under this scheme will require the support of at least 55% of the Council of the European Union members who must also represent at least 65% of the EU's citizens.
When Montenegro voted to split from Yugoslavia, the EU insisted on a supermajority of 55% for it to recognise the result. This supermajority implemented something akin to a double majority and avoided the endless debate that might result if the result had a tiny majority.
Northern Ireland 
In Romania, any kind of referendum must obtain a double majority in order to be considered valid. The first criteria for validation is the meeting of a quorum of at least 50% of the eligible voters. Only if they cast their votes, a second criteria is checked: an absolute majority of them have to agree the proposal in the referendum. If both this criteria are met, the referendum is declared valid by the Romainan Constitutional Court, which is in charge of overseeing over the entire process. After a referendum is validated, its result becomes compulsory for the representative organs like the Parliament or the President.
The independence referendum for Southern Sudan required 51% of the vote and 60% turnout.
In Switzerland, the passing of a constitutional amendment by initiative requires a double majority; not only must a majority of people vote for the amendment but a majority of cantons must also give their consent. This is to prevent a larger canton from foisting amendments onto the smaller ones and vice versa.
United States 
Double majority is used in the United States for some initiative or referendum votes on issues such as a tax levy or bond. Essentially, a double majority standard applies a two-part test to a vote outcome before a measure is passed:
- 1. Did a majority of registered voters turn out for the election? If voter turnout does not surpass this threshold, the measure fails, regardless of the outcome of those who did vote.
- 2. Did the measure pass with the requisite majority of votes? If so, the measure passes. If not, the measure fails.
This mechanism is used to prevent a small group from passing spending measures that affect the entire population in order to support their pet causes, especially at an election expected to have low voter turnout. Double majorities are also frequently used in municipal annexations, wherein majorities of both the residents in the annexing territory and the territory to be annexed must support the annexation. A similar rule exists for adopting Metro government in Tennessee, where the referendum must pass both inside and outside the principal city.
- Article 6(2) of the Lisbon Treaty.
- "Timeline: The road to Lisbon". BBC News. 3 November 2009. Retrieved 2009-11-04.
- "Sky News Australia - World News Article". Skynews.com.au. Retrieved 2011-11-20.
- "Tennessee Code Annotated 7-2-106, Referendum on proposed charter.".
- Butterworths Concise Australian Legal Dictionary, 2nd edition (2002). ISBN 0-409-31568-0
- Europa Glossary