CDL Doubles and Triples Study Guide 2026

The doubles/triples endorsement

The T endorsement needs a knowledge test only. In many states, such as Washington and Illinois, it has 20 questions, and you need 80% (16 correct). It cannot be placed on a commercial learner's permit. Study the general knowledge, air brakes and combination vehicles material as well, because these questions build on them.

Pulling doubles and triples safely

Doubles and triples are more likely to turn over than other combination vehicles. A speed that is safe for a straight truck or a single trailer may be too fast for doubles or triples.

Following distance matters even more with a long rig. Use at least 1 second for every 10 feet of length, and add 1 second above 40 mph. For example, a 70-foot double at 50 mph needs at least 8 seconds. An empty or lightly loaded trailer has less traction, so its wheels lock more easily and a jackknife is more likely. If a trailer starts to skid, release the brakes so the wheels can grip again.

The crack-the-whip effect

A quick steering move grows larger toward the back of the rig. This rearward amplification makes the last trailer the one most likely to roll over. With triples, the last trailer can roll about 3.5 times as easily as a five-axle tractor-semitrailer. Steer gently and go slowly around corners, ramps and curves.

Inspect completely. There are more critical parts to check on two or three trailers.
Look far ahead. Drive very smoothly so you can slow down or change lanes gradually.
Manage space. You cannot turn or stop quickly, so leave more following distance and wait for larger gaps.
Bad conditions. The extra length and more dead (non-driven) axles raise the chance of skids, so take extra care on slippery roads and in mountains.
Parking. Don't pull into a spot you cannot pull straight out of.
Antilock brakes. Converter dollies built on or after March 1, 1998 must have ABS, with a yellow lamp on the left side.

Coupling twin trailers

A converter dolly is a short axle unit with a fifth wheel. Its tow bar ends in a pintle eye that hooks to the pintle hook at the back of the first trailer, and the second trailer's kingpin locks into the dolly's fifth wheel.

Coupling the tractor to the first trailer. The same checks (jaws open, straight approach, kingpin locked, lines connected, landing gear up) apply when a converter dolly goes under the next trailer.

Line the tractor up straight in front of the trailer.
Check the fifth wheel: tilted down toward the back, with the jaws open.
Back slowly toward the trailer.
The kingpin locks into the fifth wheel.
Connect the air lines and the electrical cable.
Raise the landing gear all the way.

Secure the rear trailer. If it has no spring brakes, connect the emergency line, charge the trailer's air tank, then disconnect the line to set its emergency brakes. Chock the wheels if you have any doubt.
Put the heaviest trailer first. The more heavily loaded trailer goes right behind the tractor; the lighter one goes at the rear.
Position the dolly. Release the dolly brakes by opening its air tank petcock (or use its spring brake control). For short distances, wheel it by hand in line with the second trailer's kingpin.
Hook the dolly to the first trailer. Back the first trailer in front of the dolly tongue, hook the dolly, lock the pintle hook, and raise the dolly support.
Check trailer height. The second trailer must be slightly lower than the center of the dolly's fifth wheel, so it rises a little as the dolly goes under.
Back the dolly under the rear trailer. Then raise the trailer's landing gear just off the ground.
Test the coupling by pulling against the second trailer's kingpin, then check by eye: no space between the upper and lower fifth wheel, and the jaws closed on the kingpin.
Connect the safety chains, air hoses and light cords.
Set the valves. Close the dolly air tank petcock and the shut-off valves at the rear of the second trailer. Open the shut-off valves at the rear of the first trailer and on the dolly.
Raise the landing gear all the way.
Check the air. Push in the trailer air supply knob, then open the emergency line shut-off at the rear of the last trailer. If no air comes out, the brakes will not work.

To couple triples, couple the tractor to the first trailer as you would any semitrailer, then add each trailer with the converter dolly method. For other combinations, follow the manufacturer's or owner's instructions.

Uncoupling

Park the rig in a straight line on firm, level ground and apply the parking brakes. Chock the second trailer's wheels if it has no spring brakes.
Lower the second trailer's landing gear enough to take some weight off the dolly.
Close the air shut-offs at the rear of the first trailer (and on the dolly, if it has them).
Disconnect and secure the dolly's air lines and light cords, and release the dolly brakes.
Release the dolly's fifth wheel latch and slowly pull the tractor, first trailer and dolly forward to bring the dolly out from under the rear trailer.
Lower the dolly's landing gear, unhook the safety chains, set the dolly's spring brakes or chock its wheels, release the pintle hook, and slowly pull clear.

Never unlock the pintle hook while the dolly is still under the rear trailer. The tow bar can fly up and injure someone, and it will be very hard to couple again. To uncouple triples, take off the third trailer first by pulling the dolly out, then uncouple the rest like a double.

Inspection and air brake checks

Do the full seven-step inspection, plus the combination vehicle checks of the fifth wheels, air and electrical lines, and landing gear. Then add these checks for doubles and triples:

Item	Correct position or condition
Shut-off valves at the rear of front trailers	Open
Shut-off valves at the rear of the last trailer	Closed
Converter dolly air tank drain valve	Closed
Pintle hook	Latched, with the dolly's pintle eye in it
Safety chains	Secured to the trailers
Light cords and air lines	Firmly connected and supported
Spare tire on the dolly	Secured

Checking that air reaches every trailer

Hold the rig with the parking brake or chocks and let the air pressure build to normal.
Push in the red trailer air supply knob.
Apply the trailer hand brake to send air to the service line.
Go to the rear of the last trailer and open the emergency line shut-off. Listen for air, then close it.
Open the service line valve to check that service air reaches the last trailer, then close it.
If you hear no air, check that the shut-off valves on the trailers and dolly are open.

Other brake tests

Tractor protection valve: with the engine off, step on and off the brake to lower the pressure. The trailer air supply knob should pop out, usually between 20 and 45 psi. If the valve fails, a leak could drain the tractor's air and set the emergency brakes.
Trailer emergency brakes: make sure the trailers roll freely, then pull out the trailer air supply knob and pull gently forward. The brakes should hold.
Trailer service brakes: move forward slowly and apply the trailer hand valve. You should feel the trailer brakes work.