A CDL class tells you the general category of vehicle you're licensed to drive. Endorsements are the add-ons that authorize specific cargo or vehicle configurations on top of that base license. There are six standard endorsement codes recognized under federal CDL rules, and each requires its own knowledge test — and in some cases its own skills test and background check — separate from the general knowledge exam.
H — Hazardous Materials
The H endorsement authorizes transporting quantities of hazardous materials that require placarding under federal hazmat regulations. Because of the security implications, applicants must also pass a TSA background check (a security threat assessment) specifically for hazmat drivers, in addition to the knowledge test. First-time H applicants are also subject to federal entry-level driver training (ELDT) requirements that must be completed and recorded before the state will let you test.
Related: What to Expect on the CDL General Knowledge Test
N — Tank Vehicle
The N endorsement is required to drive a vehicle designed to transport liquid or gaseous materials in a permanently mounted cargo tank above a certain capacity. Notably, this applies regardless of whether what's in the tank is hazardous — hauling non-hazardous bulk liquid like milk or water in a qualifying tank vehicle still requires the N endorsement.
X — Combination Tank Vehicle and Hazardous Materials
The X endorsement combines the N and H requirements into a single endorsement for drivers who haul placarded hazardous materials in a tank vehicle — essentially, when both conditions apply at once. If you only need one or the other, you'd hold N or H individually instead.
P — Passenger
The P endorsement authorizes driving vehicles designed to carry passengers, such as buses, shuttles, and motorcoaches. Like the H endorsement, first-time P applicants fall under the federal ELDT requirement, and the skills test is administered in a vehicle representative of the passenger vehicle you intend to drive.
S — School Bus
The S endorsement is required specifically to operate a school bus, and it's always held in combination with the P endorsement, since a school bus is a passenger vehicle subtype with its own additional rules. The S knowledge test adds school-bus-specific content, such as safe loading and unloading procedures and student management. First-time S applicants are also subject to ELDT requirements.
T — Doubles/Triples
The T endorsement authorizes pulling more than one trailer at a time. It's only available to Class A CDL holders, and its knowledge test covers the added complexity of coupling multiple trailers and handling the different weight distribution and stability characteristics of a longer combination vehicle. Worth noting: even though the endorsement itself is federally defined, not every state's roads permit triple trailers, so check your state's own rules on top of the federal endorsement.
Do you need more than one?
Many working drivers hold several endorsements at once — for example, H and N together (or X in place of both) plus T, depending on the freight they haul. Because each endorsement has its own dedicated section in your state's CDL manual and its own separate test, the smart approach is to figure out exactly which endorsements your target job requires before you schedule anything, rather than guessing or adding them one at a time after the fact. Try our CDL practice questions.