Even HBO's "Silicon Valley" has caught on. John P. Johnson/HBO 
 
 I'm sitting in a San Francisco, counter-service restaurant waiting for my chopped salad, when a group of tall, white men wearing button-downs and fleece vests crowds around the cash register. 
 
 My boyfriend, who works in venture capital, spots them and says they're VCs. 
 
 "Do you know them?" 
 
 "No," he says. "The fleece vests. It's a giveaway." 
 
 It's true. The fleece vest, often in black and from outdoor clothing retailer Patagonia, has become the most quintessentially-VC item in an investor's wardrobe. 
 
 No one seems to know how the fashion faux pas turned into a staple, and yet, the fleece vest is inescapable in Silicon Valley. 
 
 For example, Reid Hoffman, cofounder of LinkedIn and a partner at Greylock (whose investment portfolio includes Airbnb, Facebook, Flickr, and Zynga), rocked a Patagonia Men's Better Sweater Fleece Vest in Harvest Moon Blue. It retails for $99 on the website. 
 
 Drew Angerer/Getty 
 
 Marc Benioff, who has led Salesforce to fund some of the largest private cloud software companies in the world, wore a Salesforce-branded black vest on stage at TechCrunch Disrupt earlier this year. 
 
 Steve Jennings/Getty 
 
 Angel investor Joanne Wilson, the "Gotham Gal," invests across all industries, from food, to gyms, to real estate. She elevated the black fleece stereotype with this high-fashion, mixed-materials vest. 
 
 Noam Galai/Getty 
 
 Business Insider spoke with several venture capitalists for this story and could not track down what brought on the vest's popularity. But we have some theories. 
 
 A practical reason why the vest might be so popular is, it gets chilly year-round in the Bay Area. 
 
 This year, San Franciscans (reluctantly) enjoyed one of the mildest summers on record. The city saw only one 70-degree day in August, meteorologists tell The San Francisco Chronicle. 
 
 As investors make trips up and down the Bay, meeting with entrepreneurs, sitting in on board meetings, and negotiating deals in eternal-spring weather, it helps to zip up. 
 
 "This is the Bay Area so layering is useful," Kim Milosevich, a marketing partner at Andreeseen Horowitz, tells Business Insider. 
 
 Andreessen Horowitz, one of the most well-known firms in Silicon Valley, orders Patagonia vests embroidered with the company's name for events. They typically order enough supplies for guests and employees. 
 
 "Our swag is mainly created as a gift for guests of our events, so we want it to be high quality and something they'd like [or] find useful," Milosevich says. 
 
 Jeff Berg, CEO and chairman of International Creative Management Ltd., and Eric Schmidt, executive chairman of Alphabet, Google's parent company, sport swag from the Allen & Company Sun Valley Conference in 2014. Scott Olson/Getty 
 
 Patagonia has another explanation. They just make darn good vests. 
 
 "I think one reason for the popularity is: Patagonia represents quality without being pretentious," Corey Simpson, a communications coordinator at Patagonia, says. 
 
 A casual, slightly pilled fleece vest may help Silicon Valley's many millionaires and billionaires feel (or at least look) grounded. 
 
 Patagonia would not disclose its corporate sales customers. But with a retail location in Palo Alto, Simpson says Patagonia fields a lot of questions about the trend — especially after its best-selling vest made a cameo in HBO's "Silicon Valley." 
 
 Donald "Jared' Dunn, played by actor Zach Woods, is the lovable and eccentric head of business development at Pied Piper. Whether he's living it up at a launch party for the fictional capital firm Bachmanity or polling TechCrunch Disrupt attendees on how to pivot the company, he dons a soft, fleece vest in Supply Green from Patagonia. 
 
 HBO 
 
 The show's production designer, Richard Royon, told The San Francisco Chronicle in 2014 that before the show aired, his team spent months surveying Silicon Valley for aesthetic inspiration. They racked up hours at Google, Facebook, Zynga, Apple, and Dell. 
 
 "Patagonia was clearly very important," Toyon said. "You walk around University Avenue and look at all the people wearing Patagonia during the day. It's kind of crazy." 
 
 The vest trend even spans from Silicon Valley to New York City's Silicon Alley. 
 
 Henry McNamara, a general partner at New York-based Greak Oaks Venture Capital, tells Business Insider he also owns a fleece vest. He's not sure why he sees so many in the wild. 
 
 "I guess I've never really thought about it," McNamara says. "I just really like vests." 
 
 Disclosure: The author is in a relationship with an employee of Andreessen Horowitz. ||||| The vest has become the unofficial uniform for Silicon Valley investors. HBO; Uniqlo; Melia Robinson/Business Insider 
 
 San Francisco's airport has a vending machine that dispenses vests — the unofficial uniform for venture capital investors — and Twitter has been poking fun all week. 
 
 But a spokesperson for the airport tells us that the machine's sales figures are no joke. 
 
 The Uniqlo-branded vending machine earns $10,000 a month on average, according to Doug Yakel, a public information officer for San Francisco International Airport. 
 
 At nearly $50 a pop, that means the vending machine sells about 200 vests per month. 
 
 It's one of the highest-earning vending machines that its leasing tenant — a third-party company that also sells electronics and other consumer goods out of vending machines — operates at the airport, according to Yakel. It's unclear what cut SFO and Uniqlo take. 
 
 "This is the first time we've had clothing available for sale from a vending machine, which we thought was very unique," Yakel told Business Insider. 
 
 Japanese retailer Uniqlo started selling clothes out of vending machines in airports and malls across the US last year. At the time, these so-called "Uniqlo To Go" machines stocked two of the most popular items from the brand's "LifeWear" collection: a thermal heat-tech T-shirt ($14.90) and a lightweight down jacket ($69.90) in an array of colors. 
 
 It appears Uniqlo added down vests for the tech set at San Francisco's airport. The Men's Ultra Light Down Vest typically retails for $49.90 online and in the vending machine. 
 
 "SFO has a down vest vending machine for visiting VCs," said Frank Barbieri, a serial entrepreneur, in a now-viral tweet that included a photo of the vending machine. 
 
 Venture capitalists can be seen sporting the practical outerwear wherever techies gather, from Blue Bottle Coffee shops to the Battery to the VC offices on Sand Hill Road. 
 
 San Francisco has chilly summers, with temperatures hovering between the mid-50s and high-60s. As investors travel across the Bay Area, meeting with entrepreneurs, sitting in on board meetings, and negotiating deals in eternal-spring weather, it helps to zip up. ||||| The vest has become the unofficial uniform for Silicon Valley investors. John P. Johnson/HBO; Uniqlo; Melia Robinson/Business Insider 
 
 San Francisco has reached peak San Francisco with the addition of a vending machine that dispenses down vests at the city's airport, and Twitter is having a field day. 
 
 "SFO has a down vest vending machine for visiting VCs," Frank Barbieri, a serial entrepreneur, said in a tweet that included a photo of the now infamous machine. 
 
 The vest has become the unofficial uniform for Silicon Valley investors. Venture capitalists can be seen sporting the practical outerwear wherever techies gather, from Blue Bottle Coffee shops to the Battery to the VC offices on Sand Hill Road. 
 
 Japanese retailer Uniqlo started selling clothes out of vending machines in airports and malls across the US last year. At the time, these so-called "Uniqlo To Go" machines stocked two of the most popular items from the brand's "LifeWear" collection: a thermal heat-tech T-shirt ($14.90) and a lightweight down jacket ($69.90) in an array of colors. 
 
 It appears Uniqlo has added down vests for the tech set at San Francisco International Airport. The Men's Ultra Light Down Vest typically retails for $49.90 online. 
 
 Uniqlo started selling clothes out of vending machines in airports and malls across the US in 2017. Uniqlo 
 
 "This is the first time we've had clothing available for sale from a vending machine, which we thought was very unique," Doug Yakel, a spokesperson for SFO, told Business Insider. 
 
 Yakel doesn't know how sales have been at this particular machine, but he said the airport has heard that "the leasing tenant is very happy with it." 
 
 Twitter has been roasting San Francisco all week long, and it's brutally funny. 
 
 Here are the best reactions: 
 
 Uniqlo did not immediately respond to a request for comment.
– It's an odd tale of vests, venture capitalists, and San Francisco: Start with this Business Insider story in 2016 declaring that the vest had "become the most quintessentially-VC item in an investor's wardrobe" in Silicon Valley. Flash forward to a different BI story from last week, taking note of a vending machine in San Francisco's airport that dispenses, yes, vests, and the social media jokes that revelation has generated. ("SFO has a down vest vending machine for visiting VCs," wrote one; "automatic vesting" wrote another.) And finally, a followup story at the site that the vendor is surely laughing all the way to the bank—with $10,000 in sales per month. The vests cost about $50, meaning that the machine from third-party vendor Uniqlo is dispensing about 200 of them per month at San Francisco International Airport. "This is the first time we've had clothing available for sale from a vending machine, which we thought was very unique," says an airport spokesman. Turns out, it's one of the most profitable machines at the airport.