Mass. Gen. Laws ch. 234A § 2
The judicial districts for purposes of this chapter shall be the counties unless modified by the supreme judicial court. The supreme judicial court may, by rule of court, define judicial districts for purposes of this chapter which differ in geographical area and population from the counties in accordance with the following principles: a judicial district shall be the geographical area encompassing a designated list of cities and towns; a judicial district may be defined for, or associated with, a single court or court location, or it may be defined for, or associated with, several courts or court locations.
Every citizen shall have the opportunity to serve as a grand and trial juror in at least one judicial district. There shall be no proscription against citizens of certain cities and towns being eligible or subject to perform grand or trial juror service in more than one judicial district.
Mass. Gen. Laws ch. 234A, § 2