Listen to the recap of our virtual book club with Because of Sex author Gillian Thomas, when we explored the struggle for justice on the job—past, present, and future.

In 1966, Ida Phillips, a single mother working as a waitress, sat down at her kitchen table and wrote a letter to then-President Lyndon B. Johnson. She told him her story: Despite her qualifications, Phillips had been told by a Martin Marietta employee not to apply for an assembly-line position at one of the construction-material company’s manufacturing plant. The job would have paid more than double what she was making as a waitress. It included a pension plan and insurance, benefits unavailable in most female-dominated industries at the time (and which since have only marginally improved.) The reason Phillips was turned away? She was a woman with a preschool child.

That letter, Phillips’ subsequent lawsuit, and her Supreme Court win would help spark a civil rights revolution in the workplace—one with consequences that reverberate today.

So opens Because of Sex: One Law, Ten Cases, and Fifty Years That Changed American Women’s Lives at Work, written by Gillian Thomas, senior staff attorney with the American Civil Liberties Union (ACLU) Women’s Rights Project. Despite its full title, though, Because of Sex goes beyond cases that helped shape workplace anti-discrimination policies, focusing on ten key women whose own lives changed the law.

Title VII of the Civil Rights Act of 1964 prohibits employment discrimination based on race, color, religion, sex, and national origin. And it was Phillips’ case, and the nine others profiled in the book, that would ultimately shape that law into one that, decades later, is an important tool in advancing gender and sex equality. As Thomas explained to Rewire in an interview, Title VII it is not just a foundational piece of civil rights legislation important for its historical effect on workplace equality. In the face of anti-transgender bathroom bills and statewide “religious liberties” legislation sweeping the country, it is a crucial tool for pushing equality forward.

Thomas’ book is organized along three key themes in employment discrimination law: pregnancy-related workplace policies, gender stereotypes in the workplace, and sexual harassment. Those themes act as an inroad toward thinking more broadly about how, in Thomas’ words, we achieve “substantive equality” in the workplace. They illustrate how early fights over promotions and workplace policies that kept women out of certain jobs due to concerns of harming their potential fertility foreshadowed the legal showdowns over contraception coverage in employee health-care plans in cases like Burwell v. Hobby Lobby and Zubik v. Burwell.

“The subject matter areas that I saw [as a researcher and employment discrimination litigator] were, number one, women’s capacity for pregnancy, and then their subsequent roles as mothers, which, historically, has played a huge role in their second-class status legally,” Thomas told Rewire. “Women of color have always been seen as workers, irrespective of whether they had children, so that’s not an entirely universal stereotype. But I think it’s pretty safe to say that generally pregnancy and motherhood have proven to be enormous conflicts in terms of what equality looks like when you have these distinct differences” in how race and gender are perceived.

Take, for instance, the case of Peggy Young and the question whether an employer can refuse to make on-the-job accommodations for pregnant employees when it does so for nonpregnant employees. Young, another one of the women featured in Thomas’ book, was a United Parcel Service (UPS) “air driver” who became pregnant. When Young told her employer she was pregnant, UPS told her they couldn’t accommodate the light-lifting recommendation made by Young’s medical providers. Instead, UPS told Young, she would have to take unpaid medical leave for the remainder of her pregnancy.

In March 2015, the U.S. Supreme Court ruled against UPS, vacating the Fourth Circuit Court of Appeals ruling that had supported UPS’ policy. The decision produced a new test for assessing pregnancy discrimination claims and sent Young’s case back to the lower courts for another look. Not long after the Roberts Court’s decision, UPS and Young settled the lawsuit, bringing an end to Young’s case.

The decision was a qualified win for advocates. The Roberts Court had accepted Young’s argument that UPS had no legitimate business reason for failing to accommodate her particular request, but the decision went short of ruling businesses must accommodate any pregnancy request.

But Because of Sex doesn’t stop at unpacking overt discrimination like the kind detailed in Young’s 2015 case or Phillips’ one in 1966. The book also takes a look at what the law has described as more “benevolent” kinds of discrimination. These include employment policies designed to “protect” women from endangering possible future pregnancies, such as prohibiting women employees from working jobs where they may be exposed to hazardous chemicals.

“It really all boils down to two issues that we are talking about in all these things,” Thomas explained, when discussing workplace policies that, employers have argued, were put in place to protect their female employees from potentially endangering a pregnancy. “One is [employers] ignoring hazards that apply to men and making women into baby-making machines. And number two is [employers] treating health effects or health hazards on the job as reasons for diminishing women’s opportunities, instead of arming women with information and assuming that they will make the right choice for themselves.”

This disconnect is most apparent in the case of United Automobile Workers v. Johnson Controls, Inc., another case Thomas highlights in her book. In 1982, the car battery manufacturer Johnson Controls sent a memorandum to all its employees that said “[w]omen who are pregnant or who are capable of bearing children will not be placed into jobs involving lead exposure or which would expose them to lead through the exercise of job bidding, bumping, transfer or promotion rights.”

The policy amounted to a demotion for many female employees and a closed door for others.

Title VII actually permits employers, in a limited context, to have employment policies that discriminate on their face, such as policies that permit churches to only hire members of the same faith. Johnson Controls argued its policy of keeping women out of certain positions due to employer concerns of health risks to future pregnancies fit within Title VII’s narrow window for permitting explicit discrimination.

The Supreme Court would eventually rule in 1991 that Johnson Controls’ policy violated Title VII because it forced female employees to have to choose “between having a child and having a job,” thereby rejecting the argument made by Johnson Control’s that a woman’s fertility—or infertility—can in most situations be considered a bona fide occupational qualification.

As Thomas noted in her book, “It was no coincidence that fetal protection politics were most prevalent in well-paid, unionized industries from which women historically had been excluded. Indeed they had been excluded precisely because they had been deemed physically unsuited for the dirty, sometimes strenuous work.”

But “in female-dominated fields, though, fetal protection policies made no business sense; they effectively would gut the workforce. That reality apparently trumped any hypothetical harm to employees’ future pregnancies,” Thomas wrote.

In other words, these policies didn’t exist in female-dominated fields.

Johnson Controls may have helped grant women the agency to determine how and when they earned a paycheck with regard to policies targeting their potential fertility, but it hardly ended the debate around when and how employers attempt to diminish women’s opportunities related to their roles as potential mothers. This has played out in the hundreds of lawsuits over the contraception benefit, for example.

In other words, if Johnson Controls had settled the question of whether a woman’s fertility was an appropriate grounds for discrimination, we would not have Hobby Lobby.

Because of Sex draws another connection between the historical fight over Title VII and the contemporary one: How do employers adjust workplace policies around shifting gender norms, and when is it discriminatory if they don’t?

The law asks, “What are women supposed to want to do?” said Thomas in her interview with Rewire. “What work are they able to do? What work do they want to do? [Given] assumptions and stereotypes that are about their abilities, their preferences, their interests and how [they are] conforming to [those] in terms of stereotypes about what femininity is—what [are] women … supposed to look and act like?”

Gender nonconforming behavior, and the manner in which employees experience discrimination as a result of that behavior, is a key component over the debate around transgender rights. But it would take a “shrill” woman and the birth of the notion of “workplace harassment” to get us and the law there first.

By every measure, Ann Hopkins should have been made a partner in the global accounting firm Price Waterhouse. She was smart. Ambitious. Worked hard and constantly outperformed her peers. But it was those very attributes that her male partners deemed “too aggressive” or as evidence that she needed “charm school,” and ultimately used to deny her a partnership that by every objective measure she had earned.

The Supreme Court would ultimately disagree. In 1989, it ruled Hopkins should have been made a partner and that the comments relating to her demeanor amounted to improper gender stereotyping, a violation of Title VII’s sex discrimination provisions.

If Hopkins was initially shut out of workplace advancement due to her defiance of feminine stereotypes, so too are women subjected to on-the-job harassment, as Thomas draws out in Because of Sex. “Sexual harassment didn’t even have a name in 1974, but was such a prevalent force driving women out of the work force, driving them into different jobs [and] subjugating them just generally in terms of the identity as sexual objects on the job,” Thomas further explained in her interview.

1974 was the year Mechelle Vinson first hired a lawyer to represent her in a case against her boss, who was chronically sexually abusing her on the job. But at the time, courts largely wrote off those kinds of complaints as a kind of chasing-around-the-office, and not sexual harassment, or in Vinson’s case, on-the-job rape. As described by Thomas in her book, “throughout the 1970s, many courts responded to complaints about abusive bosses with a collective shrug that conveyed, ‘You can’t blame a guy for trying.'”

“Sexual harassment was such a prevalent force driving women out of the workforce, driving them into different jobs, and subjugating them just generally in terms of the identity as sexual objects on the job,” Thomas told Rewire.

That “you can’t blame a guy for trying” attitude hasn’t completely gone away as far as the federal courts are concerned. After all, in 2013 the Roberts Court in Vance v. Ball State made it even harder for employees to bring workplace harassment suits, and employees still face losing jobs for “being too cute” or having their sexuality be a perceived threat to their employer’s ability to remain professional in the workplace.

Which is why, in the fight over transgender bathroom access in 2016, Title VII should be a powerful force in defeating these latest attempts to stymie social progress. The idea that “you can’t blame a guy for trying” has morphed into “how the hell can we police gender roles if we don’t know where you pee.” That’s thanks almost entirely to the manner in which the law has wrestled with gender stereotypes under Title VII, Thomas explained.

In 2012, the Equal Employment Opportunity Commission (EEOC), the federal agency charged with enforcing workplace anti-discrimination laws, issued the landmark decision Macy v. Holder, which held that employment discrimination based on transgender status was a form of unlawful sex discrimination under Title VII. Then in 2015, it issued a ruling stating that denying employees access to restrooms consistent with their gender identity is also a violation of Title VII. Meanwhile several federal courts of appeals have ruled that Title VII protects against gender identity discrimination.

But the Roberts Court has yet to weigh in.

“I think sexual orientation in a way is the sort of a final frontier” in Title VII litigation, said Thomas. “The court seems really fixated on this idea of analogizing very precisely from Hopkins. In other words, if you look or act in a way that doesn’t conform to gender stereotypes then, OK, [the courts] can understand that’s sex discrimination,” said Thomas. “But if your identity is not conforming to stereotypes in that you, you know, are romantically attracted to someone of your sex, that is harder for [the courts] to get, even though it’s obviously the most obvious manifestation of stereotype.”

This is, in many ways, a fight that started in the workplace—one that eventually got the backing of the Obama administration before becoming a flashpoint of conservative election-cycle politics. Thomas’ book doesn’t close on a prediction of what the next big Title VII fight will be per se, but it is impossible to finish it and not see the narrative threads of the historical fight for workplace equality woven throughout the the contemporary one. Sex. Gender. How the law understands and navigates the two. All this is what makes Thomas’ Because of Sex the closest thing to an assigned reading I can make.