It's sunglasses weather across much of the northern hemisphere, and yet wearing them indoors is likely to make you an object of ridicule. Why?
If you want people in a room instantly to judge you a colossal, thundering ninny, there's a very simple series of steps you can take.
Make sure you're indoors. Slip on a pair of Ray Bans. Adopt an otherworldly, rock star-like countenance. Await reaction.
Since the first mass-produced sunglasses were sold in Atlantic City, in the US, by Sam Foster 85 years ago, darkened spectacles have been widely considered stylish.
But keep them on for more than 30 seconds after you step inside a building and the effect is dramatically reversed.
"It just looks silly, doesn't it?" says cultural commentator Peter York. "It's this very early 80s idea of sophistication. Plus it's quite an impractical thing to do - you might fall over."
To get this out of the way, there are a couple of very valid reasons for wearing sunglasses indoors.
Some blind people have historically chosen to wear sunglasses. Few would quibble with the likes of Ray Charles and Stevie Wonder sporting them.
And people whose eyes are photophobic - extremely sensitive to light - may find shades alleviate their symptoms, as might some who experience migraines, says Karen Sparrow, Head of Professional Development at the Association of Optometrists.
"It's a classic way of hiding an eye deformity or a black eye," she adds. Those who suffer from dry eyes, or whose pupils have been dilated in the course of a medical examination, are sometimes advised to shield them from glare.
But for the vast, vast majority, wearing sunglasses indoors is an affectation. And it's one that can draw a strong reaction from bystanders.
Type "sunglasses indoors" into Google and you will be met by an online avalanche of vituperation. The US comedian Larry David once suggested there were two types of people who adopted this style - those who were blind, and… Well, suffice to say, those of whom Larry David did not approve.
But lots of celebrities do it.
Anna Wintour, Jack Nicholson, Bono, Lady Gaga, Victoria Beckham, Kanye West, Karl Lagerfeld - each wears tinted eyewear in all conditions.
But it's one thing to defy ridicule when you are an Oscar-winning actor, a stadium-filling pop star or the editor-in-chief of American Vogue. It's quite another when you're keeping on your shades to attend a foam party in Nuneaton.
It's easy to see why the custom is so widely disparaged. For a start, it suggests that you are trying too hard - which is the antithesis of cool.
Unnecessarily obscuring one's eyes from others is also widely considered the height of rudeness, so much so that in 2008, the etiquette guide Debrett's issued an injunction against ever wearing dark glasses inside. "It's physically shutting off the windows to your soul," says author Mark Mason, who bemoaned the prevalence of sunglasses in a recent Spectator article.
But what's most potent is the idea that this is, fundamentally, a deeply gauche and pretentious thing to do.
"People do it because they think it makes them look like Jack Nicholson," adds Mason. "They are the sort of people who pay to get into the VIP section of a nightclub. It's the equivalent of the pullover knotted around your chest or the sleeves rolled up on your jacket."
As a result, the look has somewhat shabby, unwholesome connotations.
"Sunglasses indoors, par for the course," croons Alex Turner of Arctic Monkeys mournfully on AM, the band's most recent album. He's describing an encounter in a seedy, soulless nightclub with "lights on the floor and sweat on the walls, cages and poles" - but it's the shades that tell us everything we need to know about the setting.
And yet at one stage the practice was seen as the apex of sophistication.
The photographer Louise Dahl-Wolfe did much to popularise sunglasses as a fashion accessory in Harper's Bazaar from 1938 onwards, says Vanessa Brown, author of the forthcoming Cool Shades: The History and Meaning of Sunglasses. Dahl-Wolfe's models kept theirs on both indoors and out. The accessory suggested travel, glamour, Hollywood starlets avoiding the gaze of their admirers or the flash and pop of paparazzi cameras.
Around the same era, jazz musicians like Miles Davis and Thelonious Monk sometimes used their shades during club performances to signal their refusal to engage with the racism around them and the "square" values of their audiences, Brown says.
White musicians like Lou Reed and Debbie Harry subsequently co-opted this look in dimly-lit basement gigs. There was little dissent when Dylan wore aviators to receive the Presidential Medal of Freedom inside the White House in 2012.
But not everyone could pull the look off. By the latter part of the 20th Century, thanks perhaps to the Blues Brothers, it had become a cliche, at least when adopted by non-artists.
"Musicians have earned the right to wear them indoors because they've reached another level," adds Brown, a senior lecturer in cultural studies at Nottingham Trent University. Additionally, they are often performing in front of bright lights.
Non-rock stars hope that by shutting off the windows to the soul they will assume an aura of mystery, or tap into popular culture's veneration of all that is hidden and the underground, says Brown. "But anyone who wears them in a face-to-face setting is just saying, 'I don't care about you.'"
People do try and establish non-medical justifications. Wintour described hers as "armour" which come in extremely useful sitting alongside the catwalk - "if I am bored out of my mind, nobody will notice", she told a journalist. Bono once summed up his appearance thus: "Rock star with them on, ordinary bloke without them" (in a 2005 interview, he a also said his eyes were adversely affected by flashes and bright light).
More prosaically, people who wear prescription lenses might prefer keeping on their shades to going to the bother of swapping for a pair of clear spectacles, especially if they will have to check a railway display board or the menu in a fast food restaurant.
Brown suggests that wearing shades indoors might have become so uncool it has gone full circle and emerged again as a cool thing to do, albeit equipped with an ironic, geek-chic sensibility.
"It could be deliberately uncool - saying, 'I hadn't realised it's cool, so I can't be cool,' or 'I'm so cool that I don't care if people think I'm trying to be cool.'"
Perhaps this will be the look of subsequent summers. For now, unless you have a relevant eye complaint, it's a fairly sure-fire way to be marked down as a bit of a twerp.
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