Massachusetts bars UM/UIM stacking by statute and makes underinsured coverage optional
Massachusetts's rules are in G.L. c. 175, section 113L: paragraph (1) requires uninsured motorist coverage, paragraph (2) is the elective underinsured coverage, paragraph (4) protects your recovery until you are fully compensated, and paragraph (5) bars stacking. The minimum limits are in c. 90, section 34A, the pain and suffering threshold in c. 231, section 6D and the deadline in c. 260, section 2A.
Read how stacked and single-policy coverage differ in Massachusetts, check what paragraph (5) says, or see how Massachusetts UM and UIM work.