Derivatives (of any order) of primary fields are called descendant fields. Their behaviour under conformal transformations is more complicated. For example, if O is a primary field, then π π f ( ∂ ∂ μ μ O ) ( x ) = ∂ ∂ μ μ ( π π f ( O ) ( x ) ) is a linear combination of ∂ ∂ μ μ O and O . Correlation functions of descendant fields can be deduced from correlation functions of primary fields. However, even in the common case where all fields are either primaries or descendants thereof, descendant fields play an important role, because conformal blocks and operator product expansions involve sums over all descendant fields.