Mass. Gen. Laws ch. 43 § 120
The legislative powers of the city shall be vested in a city council. One of its members shall be elected by the council annually as its president. In cities having five wards, the council shall be composed of seven members, one from each ward and two at large; in cities having six wards, the city council shall be composed of nine members, one from each ward and three at large; in cities having seven wards, the city council shall be composed of eleven members, one from each ward and four at large; in cities having eight wards, the city council shall be composed of eleven members, one from each ward and three at large; in cities having nine wards, the city council shall be composed of thirteen members, one from each ward and four at large; in cities having ten wards, the city council shall be composed of fifteen members, one from each ward and five at large; in cities having eleven wards, the city council shall be composed of fifteen members, one from each ward and four at large; in cities having twelve wards, the city council shall be composed of fifteen members, one from each ward and three at large. Each ward councillor shall be a registered voter in the ward from which he seeks election, and shall be elected by and from the qualified voters of that ward. The councillors-at-large shall be elected by and from the qualified voters of the city.
All members of the city council whether elected at large or by wards shall be elected to serve for two years from the first Monday in January following their elections and until their successors are qualified.
Mass. Gen. Laws ch. 43, § 120