  When an interface connects two CFTs, the entanglement entropy between the two
CFTs is determined by a quantity called the effective central charge. The
effective central charge does not have a simple form in terms of the central
charges of the two CFTs, but intricately depends on the transmissive properties
of the interface. In this article, we examine universal properties of the
effective central charge. We first clarify how the effective central charge
appears when considering general subsystems of the interface CFT. Then using
this result and ideas used in the proof of the $c$-theorem, we provide a
universal upper bound on the effective central charge.
  In past studies, the effective central charge was defined only in two
dimensions. We propose an analogue of the effective central charge in general
dimensions possessing similar universal properties as in two dimensions.
