Here is a definitive site:
http://www.nhtsa.dot.gov/people/injury/enforce/cvm/CMV_license.html
Class A -- Required for a combination vehicle with a gross combination weight rating (GCWR) of 26,001 pounds or more and towing a vehicle with a gross vehicle weight rating (GVWR) of more than 10,000 pounds.
Class B -- Required for a single unit vehicle with a gross vehicle weight rating (GVWR) of 26,001 pounds or more, or any such vehicle towing a vehicle not in excess of 10,000 lbs. GVWR.
Class C -- Any vehicle, regardless of weight rating, transporting hazardous material in any amount requiring the display of placards by 49 CFR, part 172, subpart F, or designed to transport 16 passengers or more, including the driver.
This is a confusing issue...so confusing that when I called 5 Highway Patrol Posts earlier this year, I got 4 different answers. Bottom line...you could be cited by LEO who doesn't understand the CDL requirements any better than most of us. The most common misunderstanding seems to be that ANY trailer with a GVWR of 10,001# or more necessitates a CDL for the towing vehicle's driver. Not so. Read carefully....A. any COMBINATION of truck/trailer with a combination GCWR of 26,001 or more AND towing a trailer with a GVWR of more than 10,000. So if your truck is GVWR of 20,000, and your trailer 7,000 your combination is over 26,000, BUT your trailer is not more than 10,000...so no CDL is necessary. Nor is a CDL necessary if your truck GVWR is 10,000 and your trailer's is 14,000...because your combination is NOT OVER 26,000. Now if your truck is 12,500 GVWR and your trailer is 14,000 GVWR...you need a CDL.
B. Any single unit vehicle with a gross vehicle weight rating (GVWR) of 26,001 pounds or more, or ANY SUCH VEHICLE towing a vehicle not in excess of 10,000 lbs. GVWR.....(meaning if your single unit vehicle has a GVWR UNDER 26,000, you are not subject to condition B)
C. Seems much more clear and has little application in this forum.
In Ohio, the GVWR is taxed and listed on the truck plate (for commercial plates). Commercial trailers, regardless of size or GVWR, are all taxed and licensed the same. This is interesting, because your licensed GVWR could be 35,998 (truck 25,999, trailer 9,999) and you MIGHT not need a CDL...but the LEO could not tell because the vehicle/trailer GVWRs are not separated. We have a Ford SuperDuty 250 licensed at 24,000 (no, we have never pulled that much with a 250, but I'd rather pay the tax than the fine for being overweight). We pull a 7,000 tagalong and a 14,000 gooseneck...both have the same kind of license plate, no weight on the registration.
Best bet...keep a copy of the requirements with you, be pleasant, and be prepared to possibly be cited in error.
Tie 'em down good and haul safe.