Optional Preferential Voting (OPV) is a system of vote-casting used in the states of Queensland and New South Wales in the Commonwealth of Australia. Most Australian elections are run under strict rules of preferential voting, where all candidates must be numbered in order of the preference of the voter, or the vote will not be counted. Although complete numbering is not required under the OPV system, a single-preference vote must still use a '1' and not a tick or cross in order for the cast vote to be considered valid.