This is the king of all boundary lines and the boundary line most often disturbed by land surveyors, thereby pitting neighbor against neighbor and plummeting the parties into civil litigation and lifelong hatred. And this is the least understood boundary line, especially by land surveyors, the ones licensed by the state to make boundary line determinations. The coterminous landowners understand this boundary line very well, at least until a land surveyor comes along and tells them that the line is in some other location.
Stopping in Arizona first, we will see that although boundary by acquiescence has been acknowledged by the Arizona courts, the elements had not been clearly defined until Mealey v. Arndt. In this case we see the Arizona court reach out to other state jurisdictions in order to apply the elements they needed to resolve their case.