For the first time since it moved to a computerized voting system in 1997, the city of Cambridge will hold a recount after last week’s closely contested school committee election.

The Cambridge Board of Election Commissioners voted unanimously yesterday to certify petitions from three candidates contesting last week’s election and seeking a recount.

Just seven votes separated school committee candidates Susana M. Segat, Richard Harding Jr. and Nancy Walser. In the official tally after election day, Harding and Walser won seats, while Segat, a three-term incumbent, lost hers.

With such a small margin separating the candidates, it is very possible that the recount could change the result of the the election.

The recount will begin Nov. 26 at 9 a.m. at the board’s old offices on Green Street.

Complicating the recount process, Cambridge uses a proportional representation (PR) system of voting, which allows voters to assign weighted ranks to multiple candidates.

Once a candidate fulfills the quota of votes needed to win, the extra votes are reallocated to the next candidate. Because the extra transfer votes are randomly chosen and may rank different candidates second, they could change in a recount, altering the outcome.

“It’s like a 17,000-piece puzzle,” said Election Commissioner Lynne A. Molnar.

The overall tenor of the board meeting was calm, in stark contrast to the partisan wrangling during the Florida recount in last fall’s presidential election. Attorneys for the three petitioning candidates asked many of the same questions and made similar comments to the board.

Segat, Harding and Walser hold similar views on the major issues, another difference from the Florida recount. The progressive Cambridge Civic Association (CCA) has endorsed all three.

The board unanimously passed a motion appointing Theresa S. Neighbor, the executive director of the Election Commission, the administrator of the recount. Neighbor and the recount’s auditor will put together a plan to conduct the recount by Nov. 21, when the board holds its next meeting.

The city’s guidelines are not very specific, leaving officials some leeway in determining procedures.

Election Commission Chair Rusty Drugan said that time and cost would be a consideration.

“We don’t have an unlimited budget to do whatever we might like,” Drugan said.

Before the recount can begin, officials must decide which ballots to include in the count. Ballots that were originally included but were challenged could be excluded from the recount if determined ineligible. Conversely, ballots that were originally escrowed—cast but not counted, pending verification—could be included in the recount of the voters who cast them are determined to be eligible. Rejected absentee ballots and rejected absentee ballot applications could also change the status of individual ballots.