"Glad-hand connectors" (also known as "palm couplings") are air hose connectors, each of which has a flat engaging face and retaining tabs. The faces are placed together, and the units are rotated so that the tabs engage each other to hold the connectors together. This arrangement provides a secure connection but allows the couplers to break away without damaging the equipment if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines. These connectors are similar in design to the ones used for a similar purpose between railroad cars. Two air lines typically connect to the trailer unit. An "emergency" or "main" air supply line pressurizes the trailer's air tank and disengages the emergency brake, and a second "service" line controls the brake application during normal operation.