We give local axioms that uniquely characterize the crystal-like structure on shifted tableaux developed in [8]. These axioms closely resemble those developed by Stembridge [18] for type A tableau crystals. This axiomatic characterization gives rise to a new method for proving and understanding Schur Q-positive expansions in symmetric function theory, just as the Stembridge axiomatic structure provides for ordinary Schur positivity.This result gives a method to show that a symmetric function is Schur-positive: one introduces operators e i , f i on the underlying set, satisfying the local axioms. Theorem 1.1 then implies that the weight generating function is Schur-positive, and enumerating the highest weight elements of the connected components gives the coefficients in the Schur expansion. This method has recently been applied successfully by Morse and Schilling [13] to certain affine Stanley symmetric functions.In this paper, we give analogous local axioms for the crystal-like structure introduced in [8] on shifted semistandard tableaux. We write = ShST(λ/µ, n) for the set of shifted semistandard tableaux of shifted skew shape λ/µ and entries ≤ n. The raising and lowering operators defined in [8] were introduced to answer geometric questions involving the cohomology of the odd orthogonal Grassmannian H * (OG(n, V )), where V is a (2n + 1)-dimensional complex vector space with a nondegenerate symmetric form [9]. Combinatorially, these operators are coplactic, that is, their action is invariant under shifted jeu de taquin slides, and this property essentially determines their action on all shifted skew semistandard tableaux (see Sections 2 and 5 for additional discussion).Interestingly, although the crystals' combinatorial and enumerative properties describe type B Schubert calculus, the crystal itself resembles a 'doubled' type A crystal, with two sets of operators e i , f i and e i , f i . Accordingly, our axioms resemble a 'doubled' form of the Stembridge axioms.Remark 1.2. In type B, there are two related classes of tableaux, 'P-tableaux' and 'Q-tableaux', enumerated respectively by the Schur Pand Q-functions. A crystal structure on P-tableaux, corresponding to the representation theory of the quantum queer superalgebra q(n), was introduced in [6], and the combinatorics and structure of these crystals were further studied in [1], [7], and [10]. In [5], Choi and Kwon use the structure to understand Schur P-positivity of certain skew Schur functions.The crystals studied in this paper are on Q-tableaux and are nonisomorphic to the q(n) crystals. The Schur Pand Q-functions, first defined by Schur [16], are dual classes of symmetric functions under the standard Hall inner product [12], and it would be interesting to understand whether these crystals are also in some sense 'dual' to each other.Our main theorem is as follows. Theorem 1.3. Let G be a directed graph with vertices weighted by n ≥0 and edges labeled by 1 , 1, . . . , (n−1) , n−1. Suppose G satisfies the axioms stated in section 3 below.Then every connec...