Edith "Edie" Windsor is challenging the Defense of Marriage Act and the tax implications for gay couples. (Andy Ryan/ANDY RYAN)

A brooch sealed it. A simple circle of diamonds that meant one thing to the world, but something entirely different to Edie and Thea.

It couldn’t be any other way. Not back then, not before Stonewall, not before the sight of two women walking hand-in-hand became almost ho-hum instead of “Oh, wow!”

No, on that day in 1967, the only option Thea Spyer and Edie Windsor could envision was the one they opted for: an engagement that was theirs and theirs alone, a bond formalized not overtly by rings on their left hands but subtly by a brooch pinned close to Edie’s heart.

Thea pulled the car over to offer it. She dropped to a knee, but Edie was spouting, “Yes, yes, yes” before Thea could finish asking the question. “We were really very much in love,” Edie, now 83, says in an interview, her delight about that moment undiminished by the passage of more than four decades.

What happened that day in the driveway of a Hamptons beach house is as wrapped in complexity as it was a generation ago, though the contours have shifted right along with the evolution of attitudes and laws about same-sex couples. Today the question isn’t whether Thea and Edie could marry — as they finally did in 2007 — but how that marriage should be treated for tax purposes. It is a question that a phalanx of civil rights attorneys is asking the U.S. Supreme Court to answer in a case that they hope will alter federal law, specifically the much-debated Defense of Marriage Act, or DOMA.

When Thea died in 2009, Edie was presented with a gargantuan tax bill — more than $360,000, a figure derived primarily from the vast increase in value of the two homes — an apartment in Manhattan and a small weekend place in the Hamptons — that they’d purchased long ago. In a heterosexual marriage, a surviving spouse can usually shield up to $5 million worth of assets from the estate tax (the limit was $3.5 million when Thea died). But that deduction is not given to married gay couples because DOMA defines marriage as “a legal union between one man and one woman.”

Edie has already won the first round in her fight. A federal court in New York ruled last month that DOMA’s Section 3 — the portion that defines marriage — is unconstitutional and that she should be refunded the estate tax she paid.

Now her legal team — New York lawyer Roberta Kaplan joined by the American and New York Civil Liberties Union foundations — is looking to make the case a part of American legal history by presenting it to the Supreme Court, which is also being asked to take up two other big DOMA cases. Their opponent is formidable — the Bipartisan Legal Advisory Group, or BLAG — a congressional body helmed by famed lawyer and former George W. Bush administration solicitor general Paul Clement. The congressional entity handles DOMA cases because the Obama administration has said it will no longer defend the law.

In this legal contest, the two sides seem to be at war about more than a law — they seem in conflict over the central essence of the case. Clement, in his dense, potent legal filings, places Congress at the center of the case — not Thea and Edie. He argues that Congress was interested in defining marriage to ensure that children have parents of both sexes. He cites remarks by the Missouri Republican Congressman Todd Akin, who said in 2004 that “we all know from experience that kids are best off when they have a mom and a dad.”

Edie and Thea never had children. But what unfolds in the pages of Edie’s legal filings is something else that they had: a decades-long relationship that spanned a crucial period in the history of gays and lesbians in the United States. They met in 1963 at Portofino, a New York restaurant where lesbians felt free to openly congregate, and they “danced a hole through the bottom of one” of Edie’s stockings.

Thea was a clinical psychologist from a Jewish family that fled the Netherlands at the outbreak of World War II and eventually immigrated to the United States. As their relationship deepened, Thea worried that Edie would be stigmatized at the workplace (she was a high-ranking systems programmer at IBM) if colleagues learned of their engagement. So, “I lied about who I was,” Edie recounted.

Edie is a buoyant sort with thick blond hair who dresses elegantly and whose voice crackles with emotion despite the effects of age and ill health (after Thea’s death, Edie suffered a heart attack that she says was caused by a medical condition known as “broken heart syndrome”). She remembered Thea as a “violin-playing jock.” But in 1977, the couple learned that Thea had multiple sclerosis. First she started to lose her sense of balance, Edie recounted. Then it was the use of her legs. Then one of her arms. She had to learn to dance from the seat of a wheelchair.

In 1993, they became one of the first couples to register as domestic partners in New York City. Thea wanted to push back the registration because of a packed schedule seeing clients. “I have waited more than 28 years for this day, and I’m not waiting a single day more!” Edie said.

It wasn’t until 14 years later — when Thea’s health was failing — that they decided they couldn’t wait anymore. They flew to Canada for a wedding ceremony because they feared that Thea would die before they would get the chance to do so in their home state. The marriage, according to Edie’s attorneys, is recognized in New York, a state that didn’t legalize same-sex marriage until 2011.

Thea’s physical condition had deteriorated dramatically. A judge met them at an airport hotel in Toronto to perform the ceremony, the culmination of a journey that was chronicled in the documentary film “Edie and Thea: A Very Long Engagement.”

This time they didn’t have to hide. Edie placed a platinum wedding band studded with five diamonds on her own finger.

But just a few years later, when the love of her life was gone and it came time to get dressed for the announcement of the lawsuit that Edie hopes will reshape American law, she reached for another piece of jewelry, one that still sparkles in her memories: a circular diamond brooch.