Many mechanics are specialized in a particular field, such as auto mechanics, truck mechanic, bicycle mechanics, motorcycle mechanics, boiler mechanics, general mechanics, industrial maintenance mechanics (millwrights), air conditioning and refrigeration mechanics, bus mechanics, aircraft mechanics, diesel mechanics, and tank mechanics in the armed services. Auto mechanics, for example, have many trades within. Some may specialize in the electrical aspects, while others may specialize in the mechanical aspects. Other areas include: brakes and steering, automatic or standard transmission, engine repairs, or diagnosing customer complaints. An automotive technician, on the other hand, has a wide variety of topics to learn. A mechanic is typically certified by a trade association or regional government power. Mechanics may be separated into two classes based on the type of machines that they work on: heavyweight and lightweight. Heavyweight work is on bigger machines, such as tractors and trailers; lightweight work on smaller items, like car engines.