The Boston mechanism is a popular student-placement mechanism in school-choice programs around the world. We provide two characterizations of the Boston mechanism. We introduce a new axiom, respect of preference rankings. A mechanism is the Boston mechanism for some priority if and only if it respects preference rankings and satisfies consistency, resource monotonicity, and an auxiliary invariance property. In environments where each type of object has exactly one unit, as in house allocation, a characterization is given by respect of preference rankings, individual rationality, population monotonicity, and the auxiliary invariance property.