Trailers are one of the main tractor's attachments used for transport of different kind of goods and materials.Today's trailers are equipped with hydraulic, units for opening and closing trailers gate, etc.
Usually they are categorized by the number of axes:

• Semi-trailer: a single-axe trailer (only back axe) with bearing to 7.5t. A semi-trailer is normally equipped with legs, called "landing gear," which can be lowered to support it when it is uncoupled. It excels great mobility and it has low barycentre so it is well for cross country.
• Double-axe trailers: Trailers with two axes (front and back) with great transport capacity. They are usually used on farms for goods transport.
• Tandems: trailers with two axes on the back side with great transport capacity.
• Universal: trailers for different types of work.