A redistribution (sometimes called redrawing or "revision") of the geographic boundaries of divisions in a state or territory takes place when an apportionment determination results in a change in the number of seats to which a state or territory is entitled, at least once every seven years, or sooner when the AEC determines that population shifts within a state or territory have caused some seats to have too many or too few voters. The "Commonwealth Electoral Act 1918" requires that all electoral divisions within a state or territory have approximately an equal numbers of enrolled voters. The "Commonwealth Electoral Act (No. 2) 1973" reduced the allowed variation of electors in each division to 10% of the state or territory's average, down from 20%. New boundaries apply only to general elections held after the redistribution process has been completed, and by-elections are held on the previous electoral boundaries.