Mass. Const. pt. 2, ch. I, § II, art. I

[There shall be annually elected, by the freeholders and other inhabitants of this commonwealth, qualified as in this constitution is provided, forty persons to be councillors and senators for the year ensuing their election; to be chosen by the inhabitants of the districts, into which the commonwealth may from time to time be divided by the general court for that purpose: and the general court in assigning the numbers to be elected by the respective districts, shall govern themselves by the proportion of the public taxes paid by the said districts; and timely make known to the inhabitants of the commonwealth, the limits of each district, and the number of councillors and senators to be chosen therein; provided that the number of such districts shall never be less than thirteen; and that no district be so large as to entitle the same to choose more than six senators.

And the several counties in this commonwealth shall, until the general court shall determine it necessary to alter the said districts, be districts for the choice of councillors and senators, (except that the counties of Dukes County and Nantucket shall form one district for that purpose) and shall elect the following number for councillors and senators, viz.: -- Suffolk, Six; Essex, six; Middlesex, five; Hampshire, four; Plymouth, three; Barnstable, one; Bristol, three; York, two; Dukes County and Nantucket, one; Worcester, five; Cumberland, one; Lincoln, one; Berkshire, two.]