A method includes detecting first movement of a first contact on an on-screen ruler in a first direction around a pivot location; in response to the first movement: rotating the ruler by an angular amount according to the first movement; and when the ruler has entered a snap zone associated with a snap angle upon rotation by the first angular amount, rotating the ruler by a first adjusted amount to bring the ruler into alignment with the first snap angle; detecting second movement of the first contact in a second direction and the second movement satisfies unsnapping criteria; and in response to the second movement, rotating the ruler by a second adjusted amount to bring the ruler into alignment with the first contact.