referendum, referral of proposed laws or constitutional amendments to the electorate for final approval. This direct form of legislation, along with the initiative, was known in Greece and other early democracies. Today, these legislative devices are widely used in certain countries, most notably Switzerland. Their use in the United States reached a peak in the early part of the 20th cent. In the United States there are two main types of referendum—mandatory and optional. The mandatory referendum may be required by state constitutions and city charters for a variety of matters. It usually applies to constitutional amendments and bond issues, which by law have to be placed before the voters for approval. The optional referendum is applied to ordinary legislation. By the usual procedure implementation of a law is postponed for a certain length of time after it has been passed by the legislature; during this time, if a petition is presented containing the requisite number of names, the proposed legislation must be put to a vote at the next election.