The method Londoners must use to elect their mayor tomorrow is a potential source of confusion. Voters will not use the familiar first-past-the-post system. Rather, they are being asked to cast both a first-preference and second-preference vote. That means making two crosses on the pink ballot paper.

The two crosses must be for different people, or only the first will be counted. It is not compulsory to use the second-preference vote. If any candidate takes more than half the first-preference votes, he will win outright. However, this looks unlikely. Without an outright win, the two leading candidates will then have their second-preference votes added to their totals. So second preferences are likely to prove crucial.

Assuming, as poll data suggests, that Labour's Ken Livingstone and the Conservatives' Boris Johnson are the two leading candidates on first preferences but neither has more than 50 per cent, second preferences have to be counted. The other candidates will be eliminated from the contest.

In this likely scenario, the second choices made by people who put eliminated candidates as first preference will still matter a lot. For example, if you vote for Brian Paddick as first choice and Boris Johnson as second preference, then assuming Mr Paddick is eliminated from the second round, the vote for Mr Johnson will be added to the Conservative's total votes. Likewise, a vote for the Greens' Si‚n Berry as first preference and for Mr Livingstone as second preference would add to Mr Livingstone's overall total.

The possibilities for tactical voting are thus considerable. The guide we publish today, prepared by Tony Travers of the London School of Economics, shows how to use your votes to best express your views on the main candidates. But above all, it is vital that as many people as possible get out and vote. Picking the right mayor matters - and it makes no sense to deprive oneself of a voice.

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