The Northern Ireland Assembly elections will use a system called the Single Transferable Vote (STV). Using this system voters in Northern Ireland will elect six members of the new assembly for each of the 18 constituencies. They vote preferentially by putting "1" against their favourite candidate, "2" against their second favourite, and so on. They can do this for as many or as few candidates as they like.

The voting is simple, but the counting is complicated. To be elected each of the successful six need to show that they have got above the minimum threshold. With six to be elected this minimum, or quota, is only one-seventh of the total vote and so STV helps to ensure that smaller parties are given a chance.

As its name implies, STV allows for votes to be transferred. This saves them from being 'wasted' - as supporters of the system might see it. There can be two kinds of wasted vote, those which take candidates way above the quota, and those for weak candidates who have no chance of reaching the quota. So these surplus votes are transferred from elected candidates and from weak candidates to those still short of the quota figure but still with a chance of reaching it.

It means that people can support candidates who they know have little chance of winning, without thinking their vote has been wasted. They know their other preferences will be taken into account later in the process.

The system also lends strength to tactical voting. That's why, for example, Sinn Fein has urged supporters to vote for "pro agreement" candidates further down their list of preferences, even if those candidates are unionists. They would argue that that would help to marginalise Ian Paisley and the other "noes".