The PR voting system, in which the field of at least 19 will run this year, has been operational in Cambridge since 1941. In this system, voters rank the candidates one through nine, rather than simply voting for a single candidate of their choice.

The so-called “number one” votes are most important.

The quota—or number of votes necessary to win—is equivalent to one-tenth of the total number of number one votes.

After a candidate garners enough number one votes to hit quota, all additional ballots with that candidate’s name in the number one slot are “transferred” to the candidate whose name is listed in the number two slot. The transferring continues down the line until a full slate of nine candidates are elected.

Proportional representation gained popularity in the 1930s, Winters explains, institute in small towns and big cities alike. Even New York City tried the system briefly.

Proponents of the election program upheld it as a means of bringing fringe groups into mainstream politics.

But local governments began to abandon the system, Winters says, when election outcomes started becoming too unconventional for political taste.

Today, Cambridge and its so-called Plan E elections stand alone.

The plan has been contested in the years since its adoption, but Cambridge, unlike other cities, has always voted to keep the electoral program.

—Staff writer Nathan J. Heller can be reached at heller@fas.harvard.edu.