The article discusses the border between Massachusetts and New Hampshire. The author discusses how the course of the Merrimack River helped define the territory for the Massachusetts Bay colony as specified in its charter. Misunderstandings regarding the geography of the river led to border disputes between Massachusetts and New Hampshire. Captains Simon Willard and Edward Johnson were appointed to a commission by the Massachusetts General Court to establish colonial boundaries. Surveyors John Sherman and Jonathan Ince were hired to assist the commission on an expedition. The Endicott Rock, a boulder inscribed by the commissioners to mark the edge of the Merrimack River, is discussed. Additional commissions established by Massachusetts and New Hampshire to resolve disputes are noted.