The bond graph is composed of the "bonds" which link together "single port", "double-port" and "multi-port" elements (see below for details). Each bond represents the instantaneous flow of energy ("dE"/"dt") or power. The flow in each bond is denoted by a pair of variables called power variables, akin to conjugate variables, whose product is the instantaneous power of the bond. The power variables are broken into two parts: flow and effort. For example, for the bond of an electrical system, the flow is the current, while the effort is the voltage. By multiplying current and voltage in this example you can get the instantaneous power of the bond.