Blakers-Massey theorem

Statement
Let $$(X;X_1,X_2)$$ be an excisive triad, viz $$X$$ is a topological space and $$X_1$$, $$X_2$$ are subspaces such that the union of the interiors of $$X_i$$s is $$X$$). Then if $$(X_1, X_1 \cap X_2)$$ is $$(n-1)$$-connected and $$(X_2, X_1 \cap X_2)$$ is $$(m-1)$$-connected, the inclusion map $$\pi_q(X_1, X_1 \cap X_2) \to \pi_q(X, X \cap X_2)$$ is an isomorphism for $$q < m + n - 2$$ and a surjection for $$q = m + n - 2$$.