Armie Hammer as Marty Ginsburg, Justin Theroux as Melvin Wulf, and Felicity Jones as Ruth Bader Ginsburg in "On The Basis of Sex."

The allure of Supreme Court justice Ruth Bader Ginsburg is hard to resist. For “On the Basis of Sex” costume designer Isis Mussenden, the appeal was as equally rooted in a love of fashion history as well as an interest in exploring Ginsburg’s story.

“There were a lot of things that I didn’t know about Justice Ginsburg,” says Mussenden. “I’ve known [about] her forever, but I didn’t really know anything about her prior to her becoming a Supreme Court justice. So I thought, what an amazing opportunity to educate all of us. And how important to women in this day and age, with everything that’s going on, to understand that there are so many different directions in which to fight the good fight.”

While designing costumes for the film taught Mussenden more about Ginsburg’s life during law school at Harvard, through her groundbreaking gender discrimination case in 1971, the experience also shed light on the experience of her own mother’s generation. “The laws that still existed in 1970 was astounding to me, absolutely shocking. And I just thought, ‘wow, my mom lived with that,'” Mussenden says. “Understanding women’s place and what they could and couldn’t do, it was fascinating to me.”

It’s been an impressive cinematic year for telling Ginsburg’s story; “On The Basis of Sex,” which comes out on Christmas Day and stars Felicity Jones and Armie Hammer as Ruth and Martin Ginsburg, was proceeded by popular documentary “RBG.” Mussenden notes that having the documentary works in favor of the biopic — for one, many viewers will already have context needed to understand Ginsburg’s movie, and buy into the idea that yes, her husband Martin Ginsburg really was that supportive of her career.

Mussenden started her process by studying every photograph of Ruth she could find from her earliest years through present day.

“Any film where you are depicting people that are alive, for me anyway, I need to know who they were, what they wore throughout their entire life,” says Mussenden. “She knew the style, she knew the trends, to the Eighties to the Nineties, in the Seventies, so that made me understand that she did have a sense of what was happening in fashion. She was a New York woman; by osmosis women in New York are more fashionable, just by knowing the street. And she was a woman who was educated, she was running in certain circles, and she had style.”

With all of that concrete knowledge gleaned from photographs, Mussenden was then able to take creative license in her approach to dressing Jones’ for the character.

“It is always collaborative with the actor. They’re wearing it, so they have to believe it, and I’m there to help find that for them. There’s a magic moment in the fitting room when you’re just starting the film and the actor walks in and I’ll put one thing on — sometimes it could just be one dress, it could be a blouse, I did one it was a belt buckle — and all of a sudden you go, ‘There’s the character.’ And it’s a magic moment where the actor transforms into the character, and then we can start,” she says. “That’s always the goal, to find that. And then from there we go, ‘OK, now I get it, this is who they are,’ and we move on.”

With Jones, that moment occurred during a 10-hour “marathon fitting” in London.

“We hit it with the silhouette, which is what I think really worked with Felicity. It was really important, the A-line skirt and the fitted top,” says Mussenden, who peppered the lineup with block colors and the fuller skirts of the mid-Fifties New Look era. “It’s almost like a Jackie O silhouette in the Seventies, very clean lines, but powerful. She didn’t need fuss, this isn’t a costume drama — this is a story about Ruth Bader Ginsburg.”

Felicity Jones as Ruth Bader Ginsburg, Cailee Spaeny as Jane Ginsburg, and Kathy Bates as Dorothy Kenyon in "On The Basis of Sex"

Felicity Jones as Ruth Bader Ginsburg, Cailee Spaeny as Jane Ginsburg, and Kathy Bates as Dorothy Kenyon in “On the Basis of Sex.”  Jonathan Wenk / Focus Features

She used the couple’s daughter, Jane, as a generational contrast, and looking at their styles also conveys how fashion trends were shaped by war and economics. Where Ruth’s fuller clothing was a response to the aftermath of war, Jane’s tighter, shorter looks were indicative of the upheaval of the Seventies.

“I had to show through her and the students that things had changed,” says Mussenden, recalling one skirt that she designed for a scene in which Jones is playing charades. “That skirt took four yards of fabric — it was crazy. And then make something for Jane, and it takes three-quarters of a yard of fabric — and they’re the same size. In the history of fashion, how much fabric you use is often connected to economics and what’s going on in the world.”

Despite the androgynous, boyish fashion gaining popularity with the young women of the era, Ginsburg maintained her feminine fashion sense.

“She fought the fight, she is an incredible feminist, she’s incredibly feminine. That is not something that she ever loses throughout her entire life, and for my generation, that’s kind of an oxymoron, we thought. But how lovely to see it. She didn’t feel she needed to be a man. She was appropriate, but she didn’t need to dress like a man, she never has. That’s confidence.”

Justin Theroux as Melvin Wulf and Felicity Jones as Ruth Bader Ginsburg in "On The Basis of Sex"

Justin Theroux as Melvin Wulf and Felicity Jones as Ruth Bader Ginsburg in “On the Basis of Sex.”  Jonathan Wenk / Focus Features

More From the Eye:

Celebrities and Models Supporting Sustainability

Call Jungle a Band, Call Them a Music Collective — Just Don’t Call Them Famous

WWD Report Card: Pink Ladies

How a Calvin Klein Designer Started His Own Floral Design Business