The rules are you can carry one quart size bag, though many people don't follow the rules.

Also note that some quart size bags are significantly larger then some other quart bags.


3-1-1 for carry-ons = 3.4 ounce (100ml) bottle or less (by volume); 1 quart-sized, clear plastic, zip-top bag; 1 bag per passenger placed in screening bin. One-quart bag per person limits the total liquid volume each traveler can bring. 3.4 ounce (100ml) container size is a security measure.

http://www.tsa.gov/traveler-information/3-1-1-carry-ons