The question before a federal appeals court on Tuesday centered around a common hiring practice that women's rights advocates say perpetuates the gender wage gap: whether or not an employer's use of prior salary to determine wages unfairly justifies paying women less than men.

Attorneys sparred before the en banc U.S. Court of Appeals for the Ninth Circuit, which considered the case that Aileen Rizo, a math consultant in Fresno County, California, filed in 2012. Rizo sued after she discovered she was paid less than male counterparts by as much as $10,000. The county justified the pay differential based on the previous salary she made at a school in Arizona.