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Carry-on Allowance
Be sure to check new requirements from the U.S. Transportation Security Administration (TSA) regarding restrictions to items in carry-on baggage. Visit www.tsa.gov for details.

Northwest allows you to carry one piece of baggage on board the aircraft plus a purse or briefcase or laptop computer or small backpack. The carry-on baggage must be of a size and shape to allow for storage in overhead compartments or underneath the seat in front of you.

Dimensions:

Cannot exceed 45 linear inches (9 inches by 14 inches by 22 inches)
Note: Baggage not meeting these specifications must be checked and is subject to any applicable fees.

http://www.nwa.com/travel/luggage/carryon.html