The trailer air supply knob is usually _________.

The trailer air supply knob, also known as the trailer supply valve or trailer emergency valve, is commonly colored red and shaped in an octagonal form. The red color and octagonal shape are used to make it easily distinguishable and recognizable for quick identification and operation. The trailer air supply knob is typically located on the dashboard or control panel of the vehicle, within reach of the driver. It is used to control the supply of air to the trailer's air brake system, allowing the driver to apply and release the brakes on the trailer.