A popular form of specifying the small signal equivalent circuit amongst transistor manufacturers is to use the two-port network parameters known as . These are a matrix of four parameters as with the parameters but in the case of the [h parameters they are a hybrid mixture of impedances, admittances, current gains and voltage gains. In this model the three terminal transistor is considered to be a two port network, one of its terminals being common to both ports. The parameters are quite different depending on which terminal is chosen as the common one. The most important parameter for transistors is usually the forward current gain, h21, in the common emitter configuration. This is designated hfe on data sheets.