Mention the name Sonia Rykiel and you think of the French designer’s geometric red bob, striped knitwear (bras optional) but mostly, eminently wearable clothes.

Rykiel the designer died almost two years ago, but for the past four years, her fashion house has been overseen by artistic director Julie de Libran. Now, as the brand celebrates its 50th anniversary, a collection showcasing 50 years of Rykiel’s “savoir-faire” launched this weekend in Paris at the start of couture fashion week. The show featured an inventory of Rykiel’s iconic pieces - skinny knit dresses designed to look like second skin and liberate movement, alongside thick, ripped jumpers, feathered gowns in bright splashes of colour and modern twists on the tuxedo - dresses, jackets and coats. Models wore Rykiel wigs and parodic takes on the famous ‘skinny boy knit’, smiling as they walked round the mosaic’d cloisters of l’ecole des Beaux Arts. It was, the notes said, a mix of transgressive luxury and nonchalant fantasy. In short: “madly Rykiel”.

“Yes, I’m nervous, of course,” admitted De Libran, speaking from her studio a few days before the show. She described the collection as a celebration of women. “This is about women’s clothes, designed for women and by women,” she said. “People now forget the skill that goes into fashion. It is so throwaway.”

The show is not couture because Sonia Rykiel does not do couture. As Marylou Luther, the notable French fashion writer (and friend of Rykiel) said in 1967, “couture is not enough. You need a Rykiel.” Rather the show is “L’Atelier Sonia Rykiel” and aims to celebrate its “past and present” in typical wearable style. Still, in many ways it will be couture in spirit – at least with a Left Bank twist. If Yves Saint Laurent, a 1970s French contemporary, co-opted the Rive Gauche (Left Bank) for his ready-to-wear collections, then Rykiel, the clothes and the woman, was the living embodiment of that aesthetic.

Dress by Julie de Libran Julie/Sonia Rykiel. Photograph: PR Company Handout

It was struggling to find decent maternity wear that sparked Rykiel’s career, although she became well known for cultivating “Nonfashion”, as her spring/summer 1977 collection called it. Her skinny “poor boy” jumpers were politicised for their unisex shape, and worn by Audrey Hepburn.

Rykiel opened her first boutique in 1968 on Paris’s Left Bank, a stone’s throw from that year’s student-led protests. At the time, the shop – small, with three sweaters and some books in the window – was linked to events only by proximity; when the riots escalated, she closed for a while. The two events would become intertwined, the cultural and sexual revolution coming to define Rykiel’s brand.

De Libran was not born then. She did, however, grow up surrounded by Rykiel’s work. “My grandmother and my mother wore her,” she says. “They were strong women. So they wore strong clothes. I remember a great mohair cardigan which smelt of my mother, which I took everywhere.”

Classic stripes at the Sonia Rykiel Spring/Summer 2018 show at Paris Fashion Week, September 2017 Photograph: Peter White/Getty Images

De Libran, 46, speaks with a cool, slightly west coast accent and is often called the “secret weapon” at whichever house she is working – one of several creative directors who came “from the back room”. This is not a snub – see Sarah Burton at Alexander McQueen, or Alessandro Michele at Gucci. De Libran’s CV is decidedly starry for someone her age, having worked with Gianni Versace, Miuccia Prada and Marc Jacobs at Louis Vuitton. She concedes she is hired because she brings a feminine bent, and because she understands how women dress, what they need. Miuccia Prada said to her: “You’re my designer that is the most commercial.”

Meeting Rykiel at her house, she remembers “the red hair, the black silhouette, and being straightforward”, she laughs. “She said to me: ‘Julie, you really need to say no. Learn it.’ ” Straightforward advice – yet tricky to navigate, even for a woman working in womenswear. “It is so hard to use for us. So many women have to say yes,” she says. She is, by chance, reading The Handmaid’s Tale. “At first I thought I can’t believe anyone could think of this. But in America, with what’s going on, I can. It’s really frightening.”

Julie Libran, artistic director at Sonia Rykiel fashion house. Photograph: Sonia Rykiel

Rykiel denied being a feminist, and while De Libran isn’t keen to dwell on the word, on the subject of equality, “I think it is going towards that, but I don’t think we are there. If you look back to Coco Chanel and Elsa Schiaparelli – there were so many women! And yet, it is still hard for us.” It’s a sentiment crystallised in a collection called ‘Generous Sweaters’ also showing in Paris, designed by high profile non-designer women. The jumpers are a nod to Rykiel’s preferred medium.

De Libran was born in Vernègues, Provence. Her family moved to California when she was eight. “I would mix French pieces with loud American T-shirts and read French Vogue,” she recalls. Being French in California, “I could never find things I wanted to wear, so I made my own clothes. I was shy too, so it became a sort of therapy. Or self expression.” She adds: “If you are wearing a structured jacket it gives you strength. If I wear heels it gives me discipline.” As a child she would sketch people and clothes, later learning to make clothes. She enrolled at Milan’s prestigious Istituto Marangoni (alumni include Franco Moschino) and moved to Paris to study draping.

Her current season is a mix of fluid checks and tight knits. A blend of Parisian chic – “French women have this in-built” – and southern roots, “which are joyful”. Taking on the mantle of an eponymous label is daunting. “But to be here makes sense. I realised I was doing it for years, working with designers in the background. This is a woman’s house. I have done the work.”