Mass. Gen. Laws ch. 43 § 59
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 more than seven wards, the city council shall be composed of fifteen members, of whom one shall be elected from each ward by and from the qualified voters of that ward, and the remaining members shall be elected by and from the qualified voters of the city. In cities having seven wards or less, the city council shall be composed of eleven members, of whom one shall be elected from each ward by and from the qualified voters of that ward, and the remaining members shall be elected by and from the qualified voters of the city.
At the first regular municipal election held in a city after its adoption of Plan B, except as otherwise provided in this section, the councillors elected from each ward shall be elected to serve for one year, and those elected at large shall be elected to serve for two years, from the first Monday in January following their election and until their successors are qualified; and at each regular municipal election thereafter the councillors elected to fill vacancies caused by the expiration of the terms of councillors shall be elected to serve for two years.
If the plan adopted provides for elections to be held biennially, at the first regular municipal election held under the provisions of such plan and at each biennial election thereafter, all the councillors whether elected at large or by wards shall be elected to serve for two years from the first day of January following their election and until their successors are qualified.
Mass. Gen. Laws ch. 43, § 59