 Causal relations are also often assumed to satisfy diachronic locality constraints. Broadly such constraints come in two types: according to the first type of constraint causes do not act where they are not. That is, causes do not act across spatial or temporal gaps. According to the second type of constraint causal influences do not propagate infinitely fast. The two types of constraint are logically independent. Newtonian gravitational theory violates both types of constraint, but rigid body mechanics violates only the finite-speed constraint while action-at-a-distance versions of classical electrodynamics (that posit particles but not fields) satisfy the finite speed constraint but posit propagation of electromagnetic influences across spatio-temporal gaps. Classical electrodynamics as classical particle-field theory is the paradigm of a local theory: interactions among charged particles propagate at a finite speed—the speed of light—and are mediated by the electromagnetic field (Frisch 2002).