of Assembly and the Tasmanian Legislative Council, semi-optional voting is used, with a minimum number of preferences required to be expressed; but there is no requirement to complete the entire ballot paper. Elections for all other Australian lower houses use full-preferential voting. In the Victorian Legislative Council, semi-optional voting is used if a voter chooses to vote below the line. Voting above the line requires only a ‘1’ being placed in one box, and group voting tickets voting has applied since 1988. The Australian Senate voting reform of 2016 switched from full-preferential voting to optional. A minimum number is specified in the instructions on the ballot paper. Since in the past a single number '1' above the line was valid, that is still a formal vote even though voters are encouraged to number six squares. Elsewhere STV Elections in Malta, in the Republic of Ireland, and in Northern Ireland use OPV. The ranked-choice voting system used in Maine, United States, can be considered optional-preferential as voters are allowed to rank just one candidate. The system also allows voters to skip one ranking (e.g. marking a first choice and a third choice, but not a second choice). In that case, the next ranking would be advanced to the next highest ranking, but more than one skip exhausts the ballot. References Instant-runoff voting