Mass. Gen. Laws ch. 237, § 4: Actual entry; seisin; proof; right of entry

Section 4. The plaintiff need not prove an actual entry under his title, but proof that he is entitled to such an estate as he claims in the land and that he has a right of entry therein shall be sufficient to prove his seisin. No such action shall be maintained unless the plaintiff has at the time of commencing his action a right of entry into the land demanded.