In fact, modern locomotives employ two air brake systems. The system which controls the brake pipe is called the "automatic brake." This system provides service and emergency braking control for the entire train. The locomotive(s) at the head of the train (the "lead consist") have a secondary system called the "independent brake." The independent brake is a "straight air" system that makes brake applications on the head-of-train locomotive consist independently of the automatic brake, providing for more nuanced train control. The two braking systems may interact differently as a matter of preference by the locomotive builder or the railroad. In some systems, the automatic and independent applications will be additive; in some systems the greater of the two will apply to the locomotive consist. The independent system also provides a "bail off" mechanism, which releases the brakes on the lead locomotives without affecting the brake application on the rest of the train.