Here are the directions for voting that you'll read at the polls tomorrow:

"Put the figure 1 opposite your first choice, the figure 2 opposite your second choice, the figure 3 opposite your third choice, and so on. You may mark as many choices as you please."

Read that last line again. You may mark as many choices as you please.

Only in Cambridge will you be able to vote for every candidate in the race because Cambridge is the only U.S. city to retain a system of voting, known as proportional representation (PR).

Cambridge has so far resisted the temptation to convert to straight majority voting because, advocates insist, the PR system consistently ensures minority representation on the city council and school committee and at the same time prevents domination by any one political faction.

To achieve that result, however, the city annually puts up with the tedious ritual of counting votes according to the PR scheme. It takes a week or more, and to pass this time of political anxiety more easily, city pols annually turn the tabulation process at the Longfellow School into something of a social event.

While the candidates nervously attempt to predict the election's final outcome, city election commissioners and staff seal the politicians' fate, giving periodic official updates.

Here's how it works:

Number one votes are the most important, because any candidate receiving a quota of first choices (10 per cent plus one vote of the total voter turnout) is automatically declared a winner in the first stage. Traditionally, only one or two candidates reach quota on the first round. In 1979, Independent Walter Sullivan and Cambridge Civic Association (CCA) member David Sullivan managed the feat.

After any candidate who surpasses quotas is declared elected, the PR process begins to get complicated. Let's assume that this year, Walter Sullivan gets 200 more votes than the quota. A random sample of 200 of his supporting ballots are then extracted from his pile of ballots, and sorted according to the number two vote on each one. Any of the surplus ballots that are not marked with a second choice are discarded.

After all of the surplus votes from the candidates who reach quota are transferred to the second choice candidates, the election commissioners begin to work from the bottom up. The weakest candidates are declared official losers, and their supporting ballots are redistributed according to the second choice on each of these ballots.

Because the weakest candidates are taken out of the race faster than the strongest ones, the losing candidates' transfer ballots play the most decisive transfer role in the PR system.

Slowly more candidates are declared winners, based on the addition of transfer ballots, until nine new city councilors and six new school committee members have been elected. In theory the transfer process could continue indefinitely through the rankings on each ballot, but in practice, the PR counting rarely continues through the fourth or fifth ranking on each ballot.

The key thing to remember about PR voting is that you will not benefit your favorite candidate by voting for him and no one else. And you could very well help your second and third choice candidate by including them, in rank order, on your ballot.

That political fact of life spawned the city's electoral slates--groups of candidates who run as a team, hoping to pick up support from their collegues' surplus ballots. The CCA slate, a group of 13 (8 for city council and 5 for school committee) candidates who favor rent control, oppose condominium conversion, and support affirmative action, traditionally elects several members on the strength of transfer ballots in the PR system.