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CDL Doubles and Triples Practice Test 2026

The doubles/triples (T) endorsement lets you pull two or three trailers. These questions cover the extra rollover risk, coupling and uncoupling with a converter dolly, shut-off valve positions and the air brake checks.

  • Commercial vehicle
  • United States
  • 50 questions
  • Checked October 10, 2026

Quick answer

The CDL double/triple trailers endorsement knowledge test has 20 questions. Pass mark: Varies by state; usually 20 questions, with 16 correct needed (80%). It is run by Your state driver licensing agency, under federal rules set by FMCSA (49 CFR Part 383). Practise here for free with 50 questions, each with the answer and an explanation. Checked against official sources on October 10, 2026.

Read before you practiseStart with the CDL Doubles and Triples Study Guide: plain-language notes, diagrams and the numbers to remember.

  1. Question 1 of 25

    Compared with other combinations, how likely are doubles and triples to turn over?
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    Answer: D. More likely

    Doubles and triples are more likely to turn over than other combination vehicles.

  2. Question 2 of 25

    Which trailer in a double or triple is most likely to turn over?
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    Answer: D. The last trailer

    Because of the crack-the-whip effect, the last trailer is the most likely to turn over.

  3. Question 3 of 25

    Compared with a five-axle tractor-semitrailer, how much more easily can the last trailer of a triple roll over?
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    Answer: A. About 3.5 times as easily

    Triples have the highest rearward amplification. The last trailer can roll about 3.5 times as easily.

  4. Question 4 of 25

    A speed that is safe for a single trailer on a curve is not always safe for doubles. Why?
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    Answer: D. Doubles and triples roll over more easily

    A speed that is fine for a straight truck or single trailer may be too fast for doubles or triples.

  5. Question 5 of 25

    What is the crack-the-whip effect?
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    Answer: D. A quick steering move swings the rear trailer much more

    Sudden steering is amplified toward the back of the rig. This rearward amplification can tip over the last trailer.

  6. Question 6 of 25

    Why do doubles and triples need extra care in the pre-trip inspection?
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    Show answer

    Answer: C. They have more critical parts to check

    With two or three trailers, there are more parts that must be checked.

  7. Question 7 of 25

    Why should you look far ahead when pulling doubles or triples?
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    Answer: C. So you can slow down or change lanes gradually

    Drive very smoothly to avoid rollover or jackknife. Looking far ahead gives you time for gradual changes.

  8. Question 8 of 25

    How should your following distance compare when pulling doubles or triples?
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    Answer: A. Allow more following distance

    Doubles and triples are longer and cannot turn or stop suddenly, so leave more space ahead.

  9. Question 9 of 25

    Why are doubles and triples more likely to skid in bad weather?
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    Answer: B. Greater length and more dead axles mean less traction

    Be especially careful on slippery roads and in mountains. Length and extra axles raise the risk of skids.

  10. Question 10 of 25

    What should you avoid when parking doubles or triples?
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    Answer: A. A spot you cannot pull straight out of

    Plan your parking so you can pull straight through and avoid a long, difficult way out.

  11. Question 11 of 25

    Converter dollies built on or after March 1, 1998 must have which safety feature?
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    Answer: C. Antilock brakes, with a yellow lamp on the left

    Converter dollies built on or after March 1, 1998 must have antilock brakes and a yellow ABS lamp on the left side.

  12. Question 12 of 25

    Which trailer should go first, right behind the tractor?
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    Answer: C. The more heavily loaded one

    Put the heaviest trailer first, behind the tractor. The lighter trailer goes at the rear.

  13. Question 13 of 25

    Where should the lighter trailer go in a set of doubles?
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    Answer: B. At the rear

    The lighter trailer goes in the rear, and the heavier one goes first.

  14. Question 14 of 25

    The rear trailer has no spring brakes. How can you secure it before coupling?
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    Answer: B. Charge its tank, then disconnect the emergency line

    Charging the tank and disconnecting the emergency line sets the trailer's emergency brakes (if the slack adjusters are right). Chock the wheels if in doubt.

  15. Question 15 of 25

    You are not sure the rear trailer's brakes will hold. What should you do?
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    Answer: A. Chock the wheels

    If you have any doubt about the trailer brakes, chock the wheels.

  16. Question 16 of 25

    How do you release the brakes on a converter dolly that has no spring brakes?
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    Answer: A. Open the air tank petcock

    Open the dolly's air tank petcock to release its brakes. If the dolly has spring brakes, use the dolly parking brake control.

  17. Question 17 of 25

    The converter dolly is close to the rear trailer. How can you move it into place?
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    Answer: A. Wheel it by hand so it lines up with the kingpin

    If the distance is short, you can wheel the dolly by hand into line with the kingpin.

  18. Question 18 of 25

    After hooking the dolly to the first trailer, what must you do with the pintle hook?
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    Answer: D. Lock it

    Hook the dolly to the front trailer, lock the pintle hook, and secure the converter gear support in the raised position.

  19. Question 19 of 25

    Before backing the dolly under the rear trailer, how high should the trailer be?
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    Answer: D. Slightly lower than the dolly's fifth wheel

    The trailer must be a little low so it is raised slightly when the dolly is pushed under it.

  20. Question 20 of 25

    Why raise the rear trailer's landing gear slightly off the ground right after backing the dolly under it?
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    Answer: C. To prevent damage if the trailer moves

    Raise the landing gear just off the ground so it is not damaged if the trailer moves during the tug test.

  21. Question 21 of 25

    How do you test the coupling between the dolly and the second trailer?
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    Answer: C. Pull against the second trailer's kingpin

    Test the coupling by pulling gently against the second trailer's kingpin.

  22. Question 22 of 25

    During the visual check of the dolly coupling, what should you see?
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    Answer: A. No gap between the fifth wheel plates, jaws closed on the kingpin

    There should be no space between the fifth wheel surfaces, and the jaws must be closed on the kingpin.

  23. Question 23 of 25

    After coupling the dolly, what must you connect between the trailers?
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    Answer: D. Safety chains, air hoses and light cords

    Connect the safety chains, air hoses and light cords.

  24. Question 24 of 25

    After coupling doubles, what do you do with the shut-off valves at the rear of the second (last) trailer?
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    Answer: C. Close them

    Close the dolly air tank petcock and the shut-off valves at the rear of the last trailer.

  25. Question 25 of 25

    After coupling doubles, what do you do with the shut-off valves at the rear of the first trailer?
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    Answer: D. Open them

    Open the shut-off valves at the rear of the first trailer, and on the dolly if it has them, so air reaches the second trailer.

What to know before the test

  • Federal rules set the passing score for every CDL knowledge test: you must answer at least 80% of the questions correctly.
  • The doubles/triples test has 20 questions in states such as Washington and Illinois, so you need 16 correct answers.
  • The double/triple trailers endorsement needs a knowledge test only.
  • The T endorsement cannot be placed on a commercial learner's permit.
  • Converter dollies built on or after March 1, 1998 must have antilock brakes, with a yellow lamp on the left side.

Frequently asked questions

How many questions are on the doubles and triples test?

Each state sets the number, but Washington and Illinois both use 20 questions. You need 16 correct to pass.

Is there a skills test for doubles and triples?

No. The T endorsement needs a knowledge test only.

Which trailer goes first in a set of doubles?

The more heavily loaded trailer goes first, right behind the tractor. The lighter trailer goes at the rear.

Which shut-off valves should be closed?

Only the ones at the rear of the last trailer. All others must be open so air reaches every trailer's brakes.

Sources

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