Mass. Gen. Laws ch. 41 § 24
There shall be one, three, five, seven or nine assessors in every city and one, three or five assessors in every town. The assessors in every city and town shall be elected or appointed as otherwise provided by law; but as nearly one third of their number as may be shall be elected or appointed annually, each to hold office for three years and thereafter until his successor is duly elected or appointed. As soon as may be after such annual election or appointment, the assessors of every city and town shall organize by choosing one of their number as chairman and another as secretary or clerk of their board. None of the foregoing provisions shall apply to the city of Boston. In no city or town, including Boston, shall an assessor hold the office of collector of taxes or deputy collector of taxes, whether said deputy is appointed under the provisions of section thirty-seven of this chapter or section ninety-two of chapter sixty.
Mass. Gen. Laws ch. 41, § 24