For the purpose of modifying the commutativity constraints and getting the Tannakian category, one needs somewhat less, and the necessary weakening is called the standard "sign" conjecture (terminology proposed by Jannsen; cf.[3] 5.1.3 for the formulation and see [3] 6.1.2.1 for obtaining the Tannakian category):is concentrated in even (resp. odd) degrees.Equivalently, for every smooth projective X/k, the sum p + X (resp. p − X ) of the even (resp. odd) Künneth projectors on H * (X) is given by an algebraic cycle. Note that such a decomposition is necessarily unique (up to unique isomorphism).This conjecture has, in addition to the consequences in terms of the category of homological motives and the algebraicity of (Hodge or Tate) cohomology classes, also the following interesting consequence, due to André and Kahn. Recall that the numerical and the homological equivalences on algebraic cycles on projective smooth varieties are conjectured to be the same. Theorem 1.2 ([3] 9.3.3.3) Let M be an additive ⊗-subcategory of M hom (k) F and let M num be its image in M num (k) F . If the sign conjecture is true for every object of M , then the functor M −→ M num admits a section compatible with ⊗, unique up to ⊗-isomorphism.Next we turn to the main geometric objects of this paper, Shimura varieties. In this paper, we will take for k a subfield of C, and for H * the cohomology theory that sends a smooth projective variety X over k to the Betti cohomology of X(C) with coefficients in number fields F .