When one look unites the Oscars, fashion weeks across two very different cities and the legacy of two fashion superstars – Karl Lagerfeld and the American socialite Lee Radziwill – well, then we have ourselves a bona fide trend.

The real fashion takeaway from Gemma Chan’s Valentino Oscars dress wasn’t the flamingo pink, but the elongated ruffled neck. The high collar ruled the night. Olivia Colman’s stately forest-green Prada dress had a polo neck, as did the va-va-voom Tom Ford sheath worn by Jennifer Lopez. Likewise, the avant-garde structured black tunic chosen by Helen Lasichanh, the very chic wife of Pharrell Williams, and Irina Shayk’s dramatic open-back black dress. Very different gowns, but all with a to-the-chin neckline.

Olivia Colman chose a high-necked Prada dress for the 2019 Oscars. Photograph: Kevork Djansezian/Getty Images

The high-fashion high neck is snowballing momentum. When Radziwill died on the first day of London fashion week, fashionista Instagram feeds were full of images of her: in a white polo neck under a blazer on the streets of Paris, with a silk scarf wound tight and high around her neck in the front row of a New York show. And then, as if by osmosis, the same outlines seeped on to the catwalks. At Victoria Beckham’s show the first look, with a red foulard whisked around the neck above a shoulder-robed windowpane blazer, had a cool east coast chic of which Radziwill would definitely have approved. At Erdem, swan-necked polo necks under bejewelled cashmere cardigans were worn with low kitten heels. Very recently retired ballerina; very Radziwill. At Roksanda, the ultra-high necklines of evening dresses were accentuated with a collar of feathers.

American socialite Lee Radziwill. Photograph: Michel Dufour/WireImage

The death of Lagerfeld threw a long shadow – complete with distinctive high-collared silhouette – over Milan fashion week. At Fendi, where Lagerfeld designed for 54 years, the exaggerated pointed collars, starched high-rise necklines and starched bows had models adopting Lagerfeld’s signature pose of a haughtily uptilted chin on the catwalk. But the high neckline was not just a homage to Karl. At Gucci, models wore spiked chokers with polo neck sweaters or above a shirt and tie. These will not go on sale – “a little dangerous”, designer Alessandro Michele explained in his post-show press conference – but were there to emphasise the neck as the focal point of this season’s look. At Versace, the 90s-party-girl classic of a tissue-weight T-shirt under a slip dress was revived, but with polo necks wrinkling right up to the ears under lace-trimmed minis.

It is now necklines, not hemlines, that make headline news in fashion. When the economist George Taylor invented the Hemline Index in 1926, theorising that short skirts reflected a buoyant economy, his logic was that in more affluent times, women choose shorter hemlines to show off their silk stockings; when times are lean, they want longer skirts to cover up the lack of them.

Gucci’s funnel neck bomber, AW19/20, Milan fashion week. Photograph: Estrop/Getty Images

Given that “showing off your silk stockings” went out before shillings and pence – and bearing in mind that the esteemed fashion historian Valerie Steele dated the debunking of the hemline index back to 1927, when hemlines began to fall two years before the stock market crash – it is amazing how long the index has hung on as urban legend. (Also, a bit patronising: “You are only girls and can therefore only talk about the economy if we explain it in terms of pretty dresses.”) Fashion has diversified to the point where there is no dictated skirt length any more. Longer skirts have dominated catwalks for most of the past decade, but not exclusively – see Balmain – and rather than the stock market, the shift reflects a more globalised fashion economy that serves non-western cultural norms, and the growing profile of older women as customers.

The polo neck has been to this decade what the miniskirt was to the 1960s. In March 2011, Phoebe Philo put a white polo neck on her Celine runway and put the high, sleek neckline back on the map. Polo necks have been layered under crew neck sweaters, hoodies and cotton shirts. The stand-up neckline has even spawned a new hairstyle: the polo neck tuck, where hair is funnelled inside a knit, turning long hair into a quasi-bob. After reigning over us for eight years, the high neck might be expected to be on the slide – instead, it is coming into its prime. The high neck is no longer a quiet underlayer, but the main event. Those Gucci spikes won’t be coming to a high street near you any time soon, but look out for a silk scarf wound into an elaborate bow, or a starched Karl-esque evening blouse, or a collar of feathers, or a zipped-to-the-chin funnel neck bomber.

A neckline has always spoken of status. Think of the white roman collar, or dog collar, worn by priests. The old school tie. The string of pearls. The white collar, and the blue. Your dress can be whatever length you like because it’s what happens at the throat that counts. This season, it’s time to put your neck on the line.