• 22 parts
  • Checked September 2026

Parts of a Tractor Trailer

A tractor and its semi-trailer from the side, the coupling up close, and the trailer from behind.

Parts of a Tractor Trailer

Name Date

Tap any part to read, rename or leave it out

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
More options

Write the name of each numbered part.Answer key

  1. Semi-trailer
  2. Landing gear
  3. Tandem axles
  4. Side marker lights
  5. Cab
  6. Sleeper
  7. Hood
  8. Steering axle
  9. Drive axles
  10. Fuel tank
  11. Fifth wheel
  12. Kingpin
  13. Glad hands
  14. Air lines
  15. Electrical cable
  16. Rear doors
  17. Clearance lights
  18. Identification lights
  19. Taillights
  20. License plate
  21. Mud flaps
  22. Rear impact guard

Drawn for this site on 2026-09-30 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the fifth wheel and its locking jaws

All 22 parts on the diagram

Show the list

Trailer 4

  1. Semi-trailer A trailer with no front axle. Its front end rests on the tractor, which carries part of its weight.
  2. Landing gear Legs that hold up the front of the trailer. They are wound down when it is uncoupled from the tractor.
  3. Tandem axles Two axles set close together, with dual tires on each end. They carry the rear of the trailer.
  4. Side marker lights Lights on the side that show how long the vehicle is. A long trailer has one at the middle as well.

Tractor 6

  1. Cab The part of the tractor the driver sits in.
  2. Sleeper A compartment behind the cab with a bed, called a sleeper berth. It is for long trips.
  3. Hood It covers the engine. The driver raises it to check the engine before a trip.
  4. Steering axle The front axle. Its wheels turn to steer the truck.
  5. Drive axles The axles the engine turns. The most common tractor has one steering and two drive axles.
  6. Fuel tank It holds the fuel. Many tractors carry one on each side.

Coupling 5

  1. Fifth wheel The coupling plate on the tractor. The front of the trailer rests and pivots on it, and its jaws lock around the kingpin.
  2. Kingpin A pin under the trailer's front. It has a flange on its lower end. It locks into the fifth wheel.
  3. Glad hands Couplers that join the air lines from the tractor to the trailer. A rubber seal in each keeps the air in.
  4. Air lines Two hoses carry air to the trailer's brakes. They are the service line and the emergency line.
  5. Electrical cable It carries power from the tractor to the lights on the trailer.

Rear of the trailer 7

  1. Rear doors Two doors that swing open for loading. They are closed and latched before the truck moves.
  2. Clearance lights They show how wide the vehicle is. One at each side, as high as practical.
  3. Identification lights A row of lights at the top center. It marks a large commercial vehicle.
  4. Taillights Lamps at the rear corners. They show the back of the vehicle to drivers behind.
  5. License plate The trailer's own registration plate.
  6. Mud flaps They hang behind the rear wheels. They stop mud and spray from being thrown up. Also called splash guards.
  7. Rear impact guard A bar hung under the rear of the trailer. It stops a car sliding underneath. Also called an underride guard.

How this list was compiled: Every name checked on 2026-09-30 and kept only where two independent sources use it. (1) The commercial driver's license manual, read in the Texas Commercial Motor Vehicle Drivers Handbook. (2) Wikipedia, text only: Semi-trailer, https://en.wikipedia.org/wiki/Semi-trailer ; Semi-trailer truck; Gladhand connector; Mudflap. (3) US Army TM 9-8000, Principles of Automotive Vehicles, chapter 38, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 . (4) Code of Federal Regulations, title 49, part 393, https://www.govinfo.gov/content/pkg/CFR-2023-title49-vol5/xml/CFR-2023-title49-vol5-part393.xml . Rear impact guard rests on sources 2 and 4; clearance, identification and side marker lights and rear doors on 1 and 4; glad hands on 1 and 2. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's product, nothing traced. Licence: own work

How it works

A tractor-trailer is two vehicles: the tractor, with the engine and the driver, and the semi-trailer it pulls. This parts of a semi truck trailer diagram has three views.

Read the full guide

Two vehicles, one pivot

The semi-trailer has no front axle. Its front rests on the fifth wheel, a plate on the back of the tractor, and a kingpin under the trailer locks into that plate. When the two are apart, the trailer stands on its landing gear.

Lines across the gap, lights at the back

Two air lines work the trailer's brakes, and each joins the trailer at a glad hand. An electrical cable powers its lights. At the back, clearance lights mark the top corners with three identification lights between them. Below the rear doors are the taillights, the mud flaps and the rear impact guard.

  • The coupling view shows the trailer on its landing gear, the only time the kingpin is in sight.
  • The most common tractor has one steering and two drive axles.

Questions and answers

What are the parts of a tractor trailer called?

The tractor has a hood, cab, sleeper, fuel tanks, a steering axle, drive axles and a fifth wheel. The semi-trailer has a kingpin, landing gear, tandem axles, rear doors, mud flaps and a rear impact guard. Air lines and an electrical cable join them.

What connects a tractor to a trailer?

The fifth wheel and the kingpin. The fifth wheel is a plate on the tractor that the front of the trailer rests and pivots on. The kingpin is a pin under the trailer, and the jaws of the fifth wheel lock around it.

What are the legs on a semi-trailer called?

The landing gear. The legs are wound down to hold up the front of the trailer when it is uncoupled, and wound up again once the trailer is locked onto the tractor.

What are glad hands on a truck?

Glad hands are the couplers that join the tractor's two air lines to the trailer. One line is the service line and the other is the emergency line. A rubber seal in each coupler keeps the air from escaping.

What is the bar on the back of a semi-trailer?

The rear impact guard, also called an underride guard. It hangs low across the back of the trailer so that a car cannot slide underneath.

More quizzes

Show 7 more

Browse Science Quizzes