Lilly Ledbetter discovered she was underpaid one spring evening in 1998 at the start of her overnight shift as a manager at the Goodyear Tire & Rubber plant in Gadsden, Ala.

She checked her mailbox as usual and found an anonymous note. On it was scribbled her monthly pay along with the pay of three men who started the same year she did and had the same job. The men were earning 15 to 40 percent more.

"My heart jerked as if an electric jolt had coursed through my body," she wrote in her 2012 memoir, "Grace and Grit: My Fight for Equal Pay and Fairness at Goodyear and Beyond." Ledbetter had worked at Goodyear for 19 years but was never quite sure she was being paid unfairly. "I was like a wife nursing a nagging suspicion that her husband's having an affair."

Complaint too late

Lilly Ledbetter went to court, and a jury found it was "more likely than not" that she suffered pay discrimination, but the Supreme Court turned her down on appeal because she didn't complain within 180 days of when she was first underpaid.

The Lilly Ledbetter Fair Pay Restoration Act, the first law President Obama signed upon taking office in 2009, fixed that for future plaintiffs by treating each undersized paycheck as a fresh, punishable offense.

The bigger problem remains: If there's no tipster stuffing notes in their mailboxes, women don't know they're underpaid. The Paycheck Fairness Act, which passed in the House in 2009, was intended to fix that deficiency by preventing companies from retaliating against workers who inquire about pay gaps. In 2010, this bill failed a procedural vote in the Senate.