Like most US carriers, Northwest allows each passenger one piece of carry-on luggage that can fit in the overhead compartment, and one "personal" item that can fit under the seat in front of you.
The bigger piece of luggage can not exceed 45" when all 3 dimensions (length, width, height) are added together. If you are taking a suit bag on the plane, it has to fit in the overhead and not exceed 45". That does count as your one piece of carry-on luggage.
Northwest's rules can be read here