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 two new axioms; favoring higher ranks and rank-respecting invariance. A mechanism is the Boston mechanism for some priority if and only if it favors higher ranks and satisfies consistency, resource monotonicity, and rank-respecting invariance. In environments where each type of object has exactly one unit, as in house allocation, a characterization is given by favoring higher ranks, individual rationality, population monotonicity, and rank-respecting invariance.