Inbound coupling — the number of other objects/units that call into a given object. Highly afferent objects (e.g. a Rails model, Ruby's BasicObject) sit at the centre of the system; changes to them propagate everywhere, so they should change rarely and be kept general. Bellware uses afference as a structural-physics metric: every method/feature added to a highly afferent object increases the 'Velcro' surface area for unrelated code to stick to.