It's 50# by THEIR scale, which may or may not agree with yours. Do not depend on your bathroom scale (that's gotten me in trouble). They do allow you to reapportion your bags before they charge you for them, which is helpful but really slows the line down (imagine every person in line doing this). You can buy a hand scale that is made especially for weighing luggage, and it's worth it if it saves you overweight charges.
The best trick is to go with a premium frequent flyer. My dh gets 2 bags at 70# each, which makes life a lot easier...