Mass. Gen. Laws ch. 40, § 47

Each town shall have a seal, established at a town meeting, to be kept by the town clerk. Papers or documents issued from any office or board of the town may be attested therewith. Cities which had an authorized seal in use on May tenth, eighteen hundred and ninety-nine, may continue to use such seal until changed by ordinance, and other cities shall by ordinance establish a seal of the city and designate the custodian thereof.