I construct multiplicies and orientations of tangent cones to any blow-up set Z for the Seiberg-Witten equation with multiple spinors. This is used to prove that Z determines a homology class, which is shown to be equal to the Poincaré dual of the first Chern class of the determinant line bundle. I also obtain a lower bound for the 1-dimensional Hausdorff measure of Z.The infinitesimal structure of the blow-up set for the Seiberg-Witten equation Definition 3. A closed nowhere dense set Z ⊂ M is called a blow-up set for the Seiberg-Witten equation with multiple spinors, if there is a solution (A, Ψ, 0) of (1) defined over M \ Z such that the following holds:(i) |Ψ| extends as a Hölder-continuous function to all of M and Z = |Ψ| −1 (0);As explained in Remark 34 below, (ii) holds automatically provided (A, Ψ, 0) is a limit of the Seiberg-Witten monopoles with τ k → 0, τ k = 0. The arguments used in this manuscript require |Ψ| to be continuous only; The Hölder continuity of |Ψ| is needed to ensure that the results of [Tau14] can be applied. In particular, a combination of [HW15, App. A] and [Tau14, Thm 1.3] yields that the Hausdorff dimension of Z is at most one. Notice also that (i) can be replaced by a weaker condition, for example [Tau14, (1.5)].I would like to stress that no extra assumptions on the Riemannian metric on M or the regularity of Z are required in Theorem 4. In particular, viewing Z as a subset of M only, the homology class of Z may be ill defined.An interpretation of Theorem 4 is that there are topological restrictions on blow-up sets for the Seiberg-Witten equation with a fixed Spin c -structure. For example, if L is non-trivial, then Z can not be empty. Although this follows immediately from Theorem 4, this statement can be proved directly by an elementary argument. Indeed, assume that for some non-trivial L there is a solution (A, Ψ, 0) of (1) such that Ψ vanishes nowhere, i.e., Z = ∅. The equation