The federal answer
The marking rule applies to self-propelled commercial motor vehicles: the USDOT number must appear on both sides of the power unit. A trailer has no engine, so the number isn't required on it — it rides under the authority of whatever truck is pulling it.
Why fleets letter trailers anyway
Branding, mostly — a 53-foot trailer is the biggest sign your company will ever own. Fleets also add unit numbers for dispatch, and drop yards want ownership ID. All voluntary, all worth doing right.
What IS required on trailers
Depending on the trailer: conspicuity tape (the red/white reflective strips — required on most trailers over 10,000 lbs and 80+ inches wide), a VIN plate, and for some vocations empty-weight/registration info. Hazmat placards go on all four sides including the trailer when the load requires them.
Common mix-up
If the combination weight (truck + trailer) crosses a threshold, that changes what goes on the truck — see the hotshot guide for the combination-weight trap.
General information, not legal advice. Rules change — confirm current federal and state requirements with the FMCSA and your state agency before you operate. Our state summaries were verified against primary sources July 2026.
Not sure what your truck needs?
Run the free 60-second compliance check — it knows all 50 states and builds your exact decal list.
Take the free compliance check →