 Before getting to the metaphysical questions about what kinds of relations these equations represent, and what it takes for a system of equations like this to be correct, let us first focus on how these equations are used in practice. It’s important to recognize that there is meant to be a difference between structural equations and ordinary equations. The equation \(D = C\) is symmetric. It could be re-written, equivalently, as \(C=D\). In contrast, the structural equation \(D \coloneqq C\) is nonsymmetric. It tells us that the variable \(D\) is causally influenced by the variable \(C\). And this form of causal influence is not symmetric. To emphasize that these equations are not symmetric, “\(\coloneqq\)” is used, rather than “\(=\)”.