The question before a federal appeals court last week 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.

A three-judge panel in April ruled the prior salary could be used to justify the pay gap. The U.S. Equal Employment Opportunity Commission and other women's rights advocates had urged the court to reconsider that decision, saying that any validation of such a practice would institutionalize the gender pay gap. Studies show women make 80 cents on the dollar to their male counterparts and that the disparities are pervasive across industries.