The conformal block G p ( s ) ( x i ) is the sum of the contributions of the primary field O p and its descendants. It depends on the fields O i and their positions. If the three-point functions ⟨ O 1 O 2 O p ⟩ or ⟨ O 3 O 4 O p ⟩ involve several independent tensor structures, the structure constants and conformal blocks depend on these tensor structures, and the primary field O p contributes several independent blocks. Conformal blocks are determined by conformal symmetry, and known in principle. To compute them, there are recursion relations and integrable techniques.