In a Lorentzian manifold, such as a spacetime model in general relativity (which will usually be an exact or approximate solution to the Einstein field equation), congruences are called "timelike", "null", or "spacelike" if the tangent vectors are everywhere timelike, null, or spacelike respectively. A congruence is called a "geodesic congruence" if the tangent vector field formula_7 has vanishing covariant derivative, formula_8.