Mass. Gen. Laws ch. 3, § 10: Prorating of additional compensation and expenses for members filling a vacancy or who resign during a regular session

Section 10. Each member of the general court who is chosen to fill a vacancy or who resigns from the general court during a regular annual session shall be entitled to any additional compensation that a member may receive pursuant to section 9B and an allowance for expenses pursuant to section 9C, prorated for the time of that person's membership.