Now, such a matrix identifies a particular process uniquely, but for any matrix to represent a process we need to add a bit more structure. Therefore, we allow processes withthe same input/output wires to be combined into one, or summed together. If a processtheory admits string diagrams, has ONBs for every type, and has sums of processes, wecan describe sums, sequential composition, parallel composition, transpose, conjugate, andadjoint all in terms of operations on matrices. We call this the matrix calculus of a processtheory.