Each winter, it’s often the case that a specific cold-weather accessory — a rainbow-check scarf, for example — comes to define the season. This year that item appears to be a knit hat that could be described as a baby bonnet for adults. It also evokes a balaclava, leading some sellers to christen it the “balabonnet.”

The accessory, which has been embraced by a certain set of fashionable women, comes in various interpretations that range from girlie to monastic. Many styles can be tied under the chin to create a streamlined egg shape, and some have longer straps that can be wrapped around the neck like a scarf for a fully snooded look.

There are bonnets embellished with sterling-silver rings, like the version by Gemsun, a brand in New York City. Mimi Wade, a label in Los Angeles, makes a cutesy style with pointed cat ears. The hats are also sold at mall chains like Free People and Hot Topic; the latter offers a bonnet covered in tiny pink bows.

A $210 version by Pien Studios, a four-year-old label in Amsterdam, has emerged as one of the most covetable. The fuzzy hat, which the brand calls a balaclava, is made of a blend of mohair, merino wool and silk and has skinny, scarflike ties. Produced in a handful of colors, it is sold at trendy boutiques across the world, including Amomento in Seoul, Esmeralda Serviced Department in Tokyo and Carmen in Amsterdam.

Grace Hwang, an advertising creative director in Brooklyn, bought a Pien Studios hat last year at Tangerine, a multibrand store in Williamsburg. Ms. Hwang, 33, said she had noticed women in New York City wearing bonnets of various designs, and called her Pien Studios version the most versatile winter accessory that she owns. (She prefers to call it a hat-scarf, not a bonnet.)

The Pien Studios hat has an ovular shape that Pien Barendregt, the label’s founder, said was inspired by those of space-age-style hats from the 1960s. Ms. Barendregt, 30, added that she aimed for a silhouette that looked feminine compared with bulkier winter gear. “It looks really elegant if you have a super big coat; it balances it nicely,” she said.

While the label calls its hat a balaclava, Ms. Barendregt agreed it was more of a bonnet and said she had received requests to make children’s versions. When she introduced the style two and a half years ago, many women described it as nostalgic, she said, adding that bonnets are practical accessories for the cold, damp winters in Amsterdam, where she lives, because they envelop the head like a hood.

Ms. Barendregt used to knit each hat herself, she said, but she recently outsourced their production in order to fulfill the hundreds of orders she has received this winter.

Lau Frías, 30, bought a white Pien Studios hat at Bomi, a boutique in Manhattan’s SoHo neighborhood, in October. “It feels like an elevated version of the many bonnets out there,” said Ms. Frías, who works in music and lives in Brooklyn. She sees the growing interest in the accessory as indicative of women not dressing to be noticed by men, but instead “thinking about looking cute for the female gaze,” she said.

While a wide selection of bonnets is available in stores, people are also knitting their own. Several patternmakers have released D.I.Y. templates, including PetiteKnit, a Danish company popular with younger knitters, which sells the pattern for its Sophie Hood — a bonnet-scarf hybrid — for 35 Danish kroner, or about $5. An Instagram video showing a finished version of the hat has been viewed more than 16 million times since being posted in late December.

The PetiteKnit founder, Mette Okkels, 35, said the hood was designed to be a little slouchy because she thinks tightfitting bonnets look too similar to versions for babies. “I don’t feel ready for that,” she said.

Recently, at the weekly knitting events hosted by Knit Club, a yarn store in Providence, R.I., a majority of the attendees have arrived wearing bonnets of their own creation, said Lindsay Degen, the store’s owner.

“And it’s not always same people every time,” added Ms. Degen, who is also a knitwear designer. “It’s a massive thing.”

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