Free CDL Doubles/Triples Practice Test 2026

The Doubles/Triples (T) endorsement is required to pull two or three trailers. It covers coupling and uncoupling multiple trailers, inspecting them, and the unique handling and rollover risks of these long combinations.

12 questions80% to passExplanations included

Question 1 of 12

What is a converter dolly (converter gear)?

Tip: Press 1-4 to select, Enter to submit, F to flag

About the CDL Doubles/Triples Test

Adding a second or third trailer multiplies both capacity and risk, and the Doubles/Triples endorsement test reflects it. The biggest hazard is the 'crack-the-whip' effect, where a small steering input at the tractor becomes a violent swing at the rear trailer - the leading cause of rollovers in these combinations. The exam covers coupling and uncoupling multiple trailers and converter dollies in the correct order, inspecting every connection and air line, and keeping speeds low and steering smooth to prevent that rearward amplification.

Inside this test

What's on the CDL Doubles/Triples knowledge test

The Doubles/Triples knowledge test is written from Section 7 (Doubles and Triples) of the national CDL manual, the text every state's CDL manual follows. You take it only if you need the T endorsement or drive a vehicle that requires it.

Federal rules fix the pass mark at 80 percent for every CDL knowledge test, but leave the number of questions to each state, so check your own state's CDL manual for the exact count. This practice test uses all 12 questions in our Doubles/Triples bank, each checked against the manual page shown with it.

Source manual: National CDL Manual, Section 7 (Doubles and Triples)
Passing score: At least 80% correct on each knowledge test (49 CFR 383.135)
Number of questions: Set by each state: federal rules require a set number of questions, with a prescribed number from each knowledge area (49 CFR 383.133)
If you fail: The state must give you a different version of the test when you retake it (49 CFR 383.133)
Test format: Written, verbal or on a computer; it may be given in a foreign language, but no interpreter may be used (49 CFR 383.133)

Manual sections this test is drawn from

Section 7: Doubles and Triples - Pulling Double/Triple Trailers, Coupling and Uncoupling, Inspecting Doubles and Triples, Doubles/Triples Air Brake Check

Sample questions from this test, with answers

Want to test yourself first? Take the test above before reading these. Every answer below is checked against the source shown.

1. What is a converter dolly (converter gear)?
- A. A hand truck for moving freight off a trailer
- B. A coupling device with axles and a fifth wheel (correct)
- C. A small jack that holds up the trailer nose
- D. A brake valve that controls trailer air
Why: A converter dolly is a coupling device with one or two axles and a fifth wheel. It lets a semitrailer be coupled to the rear of a tractor-trailer to form a double.
Source: National CDL Manual, p.7-2 (PDF p.100), 7.2.1 Coupling Twin Trailers
2. How should the shut-off valves at the back of the LAST trailer be set for driving?
- A. Open
- B. Closed (correct)
- C. Half open
- D. Open on the service line only
Why: Shut-off valves (cut-out cocks) at the rear of trailers that tow other trailers must be open. The valves at the very end of the rig must be closed so the air stays in the system.
Source: National CDL Manual, p.6-6 (PDF p.90), 6.2.7 Shut-off Valves
3. In a set of doubles or triples, which unit is most likely to turn over?
- A. The last trailer (correct)
- B. The tractor
- C. The first trailer
- D. The converter dolly
Why: Because of the crack-the-whip effect, the last trailer in a combination is the most likely to turn over. Steer gently and slow down for curves and ramps.
Source: National CDL Manual, p.7-1 (PDF p.99), 7.1.2 Beware of the Crack-the-whip Effect
4. Where should the converter dolly be placed before coupling it to the second trailer?
- A. Beside the trailer, at a right angle
- B. Behind the rear of the second trailer
- C. Under the first trailer, beside its landing gear
- D. In line with the second trailer's kingpin (correct)
Why: Release the dolly brakes and wheel it by hand, or pull it with the tractor and first trailer, so it sits in front of the second trailer in line with the kingpin.
Source: National CDL Manual, p.7-2 (PDF p.100), 7.2.1 Coupling Twin Trailers
5. Compared with other commercial vehicles, doubles and triples need:
- A. More following distance and bigger gaps (correct)
- B. Less following distance because of weight
- C. The same space as a single trailer rig
- D. Smaller gaps since they accelerate fast
Why: Doubles and triples are longer and cannot be turned or stopped suddenly. Allow more following distance, wait for large gaps before entering or crossing traffic, and be sure the sides are clear before changing lanes.
Source: National CDL Manual, p.7-1 (PDF p.99), 7.1.5 Manage Space
6. When inspecting a converter dolly, which of these should you check?
- A. Dolly air tank drain valve left open
- B. Shut-off valves at the rear of the last trailer open
- C. Pintle hook latched and safety chains secured (correct)
- D. Safety chains taken off before the trip
Why: Check that the dolly's pintle eye is in the pintle hook and the hook is latched, safety chains are secured, light cords are firmly in their sockets, and the dolly air tank drain valve is closed.
Source: National CDL Manual, p.7-4 (PDF p.102), 7.3.1 Additional Checks (Double and Triple Trailers)