An in-line thermostat is a device that is used to regulate the temperature of a system. It is typically used in heating and cooling systems to maintain a desired temperature. The thermostat works by sensing the temperature of the system and then activating or deactivating the heating or cooling system accordingly. In-line thermostats are typically installed in the return air duct of a heating or cooling system, and are designed to provide accurate temperature control. They are typically powered by electricity, and can be programmed to maintain a specific temperature at certain times of the day. In-line thermostats are a great way to maintain a comfortable temperature in your home or office, and can help to save energy by ensuring that the system is not running unnecessarily.