One of the ways in which ranked voting systems vary is whether an individual vote must express a minimum number of preferences to avoid being considered invalid ("spoiled" or "informal"). Possibilities are: Full preferential voting (FPV) requires all candidates to be ranked Optional preferential voting (OPV) requires only one candidate, the voter's first preference, to be indicated Semi-optional preferential voting requires ranking some number greater than one but less than the total number of candidates. Ranked-voting systems typically use a ballot paper in which the voter is required to write numbers 1, 2, 3, etc. opposite the name of the candidate who is their first, second, third, etc. preference. In OPV and semi-optional systems, candidates not explicitly ranked by the voter are implicitly ranked lower than all numbered candidates. Some OPV jurisdictions permit a ballot expressing a single preference to use some other mark than the digit '1', such as a cross or tick-mark, opposite the preferred candidate's name, on the basis that the voter's intention is clear; other do not, arguing for example that an 'X' might be an expression of dislike. FPV may not be possible if write-in candidates are allowed. In a transferable-vote system like the single transferable vote (STV) or instant runoff voting (IRV), a ballot is initially allocated to the first-preference candidate but may be transferred one or more times to successively lower preferences. If there is no lower preference available when such a transfer is applicable, the ballot is said to be exhausted. In such cases FPV prevents exhausted ballots. On the other hand, it increases the risk of invalid ballots: the more numbers a voter is require to enter, the greater the opportunity for mistakes, by repeating or skipping numbers or skipping candidates. Some Australian elections which mandate FPV have reduced informal votes by adding group voting tickets "above the line" on ballot papers: these allow voters to select a complete ranking prepared by one of the parties, instead of manually entering personal preferences "below the line". Australia The terms OPV and FPV are used mainly in Australia, in relation to elections at state and territory and Commonwealth (federal) level, which use proportional representation (STV) or preferential voting (IRV). OPV is used in elections in the Australian state of New South Wales. It was used in Queensland from 1992 to 2015. and in the Northern Territory in 2016. In both the Tasmanian House