A constitutional amendment is a formal change to the text of the written constitution of a nation or state. Most constitutions require that amendments cannot be enacted unless they have passed a special procedure that is more stringent than that required of ordinary legislation. Examples of such special procedures include super-majorities in the legislature, or direct approval by the electorate in a referendum, or even a combination of two or more different special procedures. A referendum to amend the constitution may also be triggered in some jurisdictions by popular initiative. Australia and the Republic of Ireland provide examples of constitutions requiring that all amendments are first passed by the legislature before being submitted to the people; in the case of Ireland, a simple majority of those voting at the electorate is all that is required, whereas a more complex set of criteria must be met in Australia (a majority of voters in a majority of states is also necessary). Switzerland has a similar procedure to Australia's. Denmark provides an example of multiple special procedures that must be followed. After an amendment has been approved by parliament, a general election must be held; the new parliament must then approve the amendment again before it is finally submitted to a referendum. There is also a requirement that at least 40% of eligible voters must vote at the referendum in order for an amendment to be validly passed. The special procedures for the amendment of some constitutions have proven to be so exacting that either few (Australia) or no proposed amendments (i.e. Japan) have been passed over a period of several decades. In contrast, the constitution of the US state of Alabama has been amended over 800 times since 901. Successive majorities Some jurisdictions require that an amendment be approved by the legislature on two separate occasions during two separate but consecutive terms, with a general election in the interim. Under some of these constitutions there must be a dissolution of the legislature and an immediate general election on the occasion that an amendment is adopted for the first time. Examples include the constitutions of Iceland, Denmark, and the Netherlands. This method is also found in subnational entities, such as the US states of Wisconsin and Vermont. Special requirements in federations The countries with a federal structure typically require that amendments to the federal constitution be approved by some or all of the federal entities as well as by the federation as a whole. An amendment to the United States Constitution must be ratified by three-quarters of the states before it can come into effect. In Canada, there are varying requirements determined by the subject matter of the amendment. There are five different methods of amendment set out in the Constitution of Canada, each relating to different elements of the constitution. Most of these methods of amendment require the consent of various provincial legislatures, following the passage of an Act of the federal parliament. In referendums to amend the constitutions of Australia and Switzerland it is required that a proposal be endorsed not just by an overall majority of the electorate in the nation as a whole, but also by separate majorities in each of a majority of the states or cantons. In addition, if an Australian referendum specifically impacts upon one or more states, then a majority of the electorate in each of those states must also endorse the proposal.