Employers cannot use salary histories as justification for paying female employees less than male ones who perform the same work, a federal court ruled on Monday.

The ruling was made in the 9th U.S. Circuit Court of Appeals and affects employers and workers in the following states: Alaska, Arizona, California, Hawaii, Idaho, Montana, Nevada and Washington.

“The question before us is also simple: can an employer justify a wage differential between male and female employees by relying on prior salary? Based on the text, history and purpose of the Equal Pay Act, the answer is clear: No,” Judge Stephen Reinhardt wrote in the majority opinion.

Plaintiff Aileen Rizo, a math consultant who worked for the Fresno County Office of Education, originally brought the case against Jim Yovino, county superintendent, in 2012 after learning that men performing the same work as she did earned significantly more, even though she had more experience and tenure. Her originally salary upon hire in 2009 had been based on the school district’s policy of adding five percent to a candidate’s most recent salary.

Rizo lost her case in 2017 when a three-member panel of appellate judges cited a 1982 decision (in Kouba v. Allstate Ins. Co.) that using past wages to determine current salary was allowed under the Equal Pay Act. Monday’s decision overturned the 1982 ruling.

In the decision released Monday, the judges said that among “legitimate, job-related factors” on which a wage differential can be based are a person’s experience, educational background, ability or prior job performance.

The entire 11-judge panel of the court heard the Rizo v. Yovino case “to clarify the law, and we now hold that prior salary alone or in combination with other factors cannot justify a wage differential,” Reinhardt wrote. “To hold otherwise — to allow employers to capitalize on the persistence of the wage gap and perpetuate that gap ad infinitum — would be contrary to the text and history of the Equal Pay Act and would vitiate the very purpose for which the act stands.”

Yovino’s attorney told CNN that he will petition for the U.S. Supreme Court to review the decision.

The ruling came one before Equal Pay Day, the day in 2018 that women must work until for their overall wages to catch up to what men overall earned in 2017, according to observance organizers.

According to newly released data from financial technology company SmartAsset, first-line supervisors of housekeeping and janitorial workers work in a job category with one of the largest pay differences between men and women. Licensed practical nurses and licensed vocational nurses, along with physical therapists and social workers, work in job categories with some of the smallest pay differences between men and women.