- What Is the X Endorsement Combination?
- How the Two Knowledge Tests Work Together
- The 10 Content Domains You'll Be Tested On
- Registration, TSA Background Check, and Fees
- Training Requirements and the ELDT Rule
- Who Actually Needs This Combination
- Building a Study Plan Around Two Tests
- Frequently Asked Questions
- X isn't a separate exam - it's the code for holding both Hazardous Materials (H) and Tank Vehicle (N) endorsements together.
- Each knowledge test requires an 80 percent passing score under 49 CFR 383.135.
- The hazmat test is commonly 30 questions; the tank vehicle test is commonly 20 questions.
- Hazmat applicants need a TSA Security Threat Assessment, $85.25 for new/renewing applicants as of January 1, 2025.
What Is the X Endorsement Combination?
The Combination of Tank Vehicle and Hazardous Materials, printed as the "X" endorsement on a commercial driver's license, is not a single standalone credential you sign up for directly. It's the designation that appears on your CDL when you hold both the Hazardous Materials (H) endorsement and the Tank Vehicle (N) endorsement at the same time. The Federal Motor Carrier Safety Administration sets the underlying standards under 49 CFR Part 383, and the actual knowledge tests are administered by your state driver licensing agency rather than a third-party testing vendor.
Because X sits at the intersection of two separate endorsement systems, understanding it requires understanding both halves independently and how they combine. If you're still getting oriented, our overview pages on What Is X? and X Meaning break down the terminology in plain language, and What Does X Stand For? walks through exactly what each letter represents on the license.
How the Two Knowledge Tests Work Together
To carry hazardous materials in a tank vehicle, a driver must pass two distinct knowledge tests, each governed by its own section of federal regulation:
- Hazardous Materials knowledge areas defined in 49 CFR 383.121, commonly administered as a 30-question test.
- Tank Vehicle knowledge areas defined in 49 CFR 383.119, commonly administered as a 20-question test.
Both tests carry the same federally mandated passing threshold: 80 percent correct, per 49 CFR 383.135. That means on a 30-question hazmat test you need roughly 24 correct answers, and on a 20-question tank test you need roughly 16 correct answers - there's no partial credit or curve. A candidate who passes hazmat but not tank vehicle only holds the H endorsement; a candidate who passes tank vehicle but not hazmat only holds N. The X appears only after both are on record.
For a deeper breakdown of exactly what "passing" means numerically and how states round scores, see X Passing Score 2026: Exactly What You Need to Pass.
Key Takeaway
Treat this as two exams to prepare for, not one. Study time should be allocated separately for hazmat regulations and tank vehicle handling, since the content, question style, and even the tank vehicle definition itself (a tank with individual rated capacity over 119 gallons and aggregate capacity of 1,000 gallons or more) don't overlap much.
The 10 Content Domains You'll Be Tested On
Across both knowledge tests, candidates are responsible for material spanning ten domains. The hazardous materials regulations domain is consistently the heaviest content block, covering the Hazardous Materials Table, shipping papers, marking, labeling, placarding, packaging, ORM-D, accident reporting, and rules around tunnels and railroad crossings.
Domain 1: Hazardous Materials Regulations
The largest and densest domain. Candidates must be able to interpret the Hazardous Materials Table, correctly complete shipping papers, and apply marking, labeling, and placarding rules.
- Know when ORM-D packaging rules apply
- Understand reporting thresholds for hazmat incidents
- Recognize restrictions at tunnels and railroad crossings
Domain 2: Hazardous Materials Handling
Covers loading, unloading, segregation, and safe handling practices for regulated materials.
- Compatibility and segregation requirements
- Proper container inspection before transport
Domain 3 & 4: Emergency Equipment and Response Procedures
Focuses on what's carried on the vehicle for emergencies and how a driver should react if a leak, spill, or fire occurs during transport.
Domain 5: Cargo Surge Effects
Liquid movement inside a partially full tank changes vehicle handling in turns, stops, and lane changes - a concept unique to tank vehicle operation.
Domain 6: Braking Procedures for Empty, Full, and Partially Full Tanks
Braking distance and stability differ significantly depending on tank fill level, and candidates must know the adjustments each scenario requires.
Domain 7: Baffled and Compartmented Versus Non-Baffled Tanks
Baffles reduce surge by breaking up liquid movement; non-baffled tanks (common with certain liquid cargoes) surge more and demand different driving techniques.
Domain 8: Tank Vehicle Type and Construction Differences
Different tank designs - pressurized, non-pressurized, cargo tanks, and others - carry different structural and handling considerations.
Domain 9 & 10: Road Grade, Curvature, and Emergency Systems
Tank vehicles behave differently on grades and curves due to their higher center of gravity and liquid load, and drivers must know the proper use of emergency shutoff and venting systems.
For a full walkthrough of each domain with sample question framing, read X Exam Domains 2026: Complete Guide to All 10 Content Areas. If you want a condensed reference to keep handy in the final days before your test, the X Cheat Sheet 2026: One-Page Review of Must-Know Facts consolidates the highest-yield facts from all ten areas onto one page.
Registration, TSA Background Check, and Fees
Because the hazmat side of this combination involves transporting regulated substances, federal law layers a security requirement on top of the knowledge test. Every hazmat endorsement applicant - new or renewing - must complete a TSA Security Threat Assessment, which includes fingerprinting.
| Requirement | Detail |
|---|---|
| TSA Security Threat Assessment (standard) | $85.25, effective January 1, 2025, for new and renewing applicants |
| TSA Security Threat Assessment (TWIC holders) | $41.00 reduced rate for applicants with a valid TWIC in a state that accepts comparability |
| Hazmat knowledge test | Commonly 30 questions; 80 percent passing score required |
| Tank vehicle knowledge test | Commonly 20 questions; 80 percent passing score required |
| Endorsement renewal cycle | Every five years, with a new threat assessment required |
State test fees and endorsement fees on top of the federal TSA charge vary by jurisdiction, so budget for both the state-side costs and the federal background check. A full cost breakdown, including how the TWIC comparability discount works, is covered in X Certification Cost 2026: Complete Pricing Breakdown.
Training Requirements and the ELDT Rule
Since February 7, 2022, first-time applicants for the Hazardous Materials (H) endorsement must complete entry-level driver training through a provider listed on the FMCSA Training Provider Registry before they're eligible to test. This training-provider-registry requirement applies specifically to the hazmat side of the equation - the tank vehicle endorsement itself carries no equivalent ELDT mandate.
This asymmetry matters for planning: if you already hold a tank endorsement and are adding hazmat later (or vice versa), your training obligations differ depending on which endorsement is new. Details on documenting training completion, registry lookups, and how this interacts with state licensing appointments are covered in X Training and in the full eligibility rundown at X Requirements 2026: Eligibility, Prerequisites & How to Qualify.
Who Actually Needs This Combination
Drivers pursue the X combination when their job requires hauling liquid or gaseous hazardous materials in bulk tank vehicles - think fuel haulers, chemical transport, and certain agricultural or industrial liquid carriers. Since a tank vehicle is defined as one with an individual compartment rated over 119 gallons and an aggregate capacity of 1,000 gallons or more, drivers moving smaller containerized hazmat loads (which would only need the H endorsement) don't need the tank combination at all.
Employers hiring for these positions typically screen for both endorsements together precisely because tanker-hazmat routes are common in fuel distribution and bulk chemical logistics. If you're evaluating whether pursuing this combination fits your career path, X Jobs outlines the kinds of roles that typically require it, and Is the X Certification Worth It? Complete ROI Analysis 2026 walks through the tradeoffs in time, cost, and renewal obligations.
Key Takeaway
Before scheduling either knowledge test, confirm with your prospective employer whether the role actually requires the tank vehicle definition thresholds (119-gallon individual / 1,000-gallon aggregate) - some hazmat-hauling jobs don't involve qualifying tank vehicles at all.
Building a Study Plan Around Two Tests
Because X requires passing two federally distinct knowledge tests, an effective plan treats them as parallel tracks rather than one blended study effort. A reasonable structure spreads preparation across a few weeks, front-loading the heavier hazmat regulations domain early since it covers the most material (the Hazardous Materials Table, shipping papers, marking, labeling, placarding, and more), then shifting focus to the more mechanically conceptual tank vehicle domains - surge, braking, baffling, and road grade - in the final stretch.
Hazmat Regulations Foundation
- Learn the Hazardous Materials Table structure and how to read entries
- Practice shipping paper completion and hazard class identification
Marking, Labeling, Placarding & Handling
- Drill placard selection scenarios and ORM-D exceptions
- Review segregation and loading/unloading handling rules
Tank Vehicle Mechanics
- Study cargo surge, baffled vs. non-baffled tank behavior
- Compare braking procedures across empty, full, and partial loads
Emergency Systems & Full Review
- Cover emergency equipment operation and response procedures
- Take full-length practice tests for both knowledge exams
Running timed practice questions on our practice test platform for both the hazmat and tank vehicle content lets you spot which domain is dragging your score down before test day rather than after. For a more detailed week-by-week plan with domain weighting rationale, see X Study Guide 2026: How to Pass on Your First Attempt, and if you're trying to gauge overall difficulty before committing your schedule, How Hard Is the X Exam? Complete Difficulty Guide 2026 compares the demands of each knowledge test.
Frequently Asked Questions
No. X is the designation your CDL receives once you've passed both the Hazardous Materials knowledge test and the Tank Vehicle knowledge test. There is no separate combined exam.
Both the hazmat and tank vehicle knowledge tests require 80 percent correct answers, as mandated by 49 CFR 383.135. On a commonly used 30-question hazmat test, that's around 24 correct; on a 20-question tank vehicle test, around 16 correct.
If you're a first-time applicant for the H endorsement, yes - since February 7, 2022, you must complete entry-level driver training through a provider on the FMCSA Training Provider Registry. The tank vehicle endorsement has no equivalent training requirement.
The Security Threat Assessment fee is $85.25 for new and renewing applicants effective January 1, 2025. Applicants with a valid TWIC in a state that accepts comparability may qualify for a reduced rate of $41.00.
The endorsement is renewed every five years, and renewal requires completing a new TSA Security Threat Assessment along with any state-required retesting.