As a new exhibition reveals Princess Diana’s innately successful approach to public dressing, Stylist explores the old and new rules of the dress code

The dress code: a thing of both beauty and horror. For while it lets us know precisely what sort of event we’ve been invited to (black tie: gravadlax blinis and Bollinger; business dress: prawn vol-au-vents and franciacorta), it can often leave us crumpled at the bottom of our wardrobe, cast adrift among lengths of silk and cotton, weeping tears of frustrated indecision. Because, if we’re honest, who among us really knows what ‘smart casual’ means? What are the discernible, quantifiable differences between cocktail dress and black tie? And who on god’s green earth has a ball gown hanging in the back of their wardrobe anyway? It’s a sartorial minefield, and as such, we look to those who always seem to get it right, no matter the occasion.

Twenty years on from her death, Princess Diana is still the gold standard for event dressing. The new exhibition Diana: Her Fashion Story, which opens this Friday at Kensington Palace, reveals that while she enjoyed fashion and liked to test the boundaries (she was the first female royal to wear trousers to an evening event), her wardrobe was planned with military deference to the dress code.

From the shy Sloane in puff sleeves for her engagement portrait and the deceptively simple white shirt she sported in a memorable 1991 photo shoot by Lord Snowdon, to an above-the-knee, bow-fronted scarlet Lacroix and 1994’s off-the-shoulder, velvet cocktail dress that made front pages around the world, Diana knew when to wear gloves or how high a hemline could creep to maintain decorum.

It’s easy to assume that in 2017 (and unburdened by aristocratic titles and duties) we are no longer beholden to such restrictions. But there is still the deep-seated fear of being either spectacularly over- or woefully under-dressed for an occasion. And what’s considered right and wrong has morphed over time. Just a few years ago, it was considered the height of rudeness for women to wear black to a wedding. Now, not only is black acceptable, we’re debating whether it’s OK to wear a white lace Self-Portrait dress to gay nuptials (a conundrum faced by a Stylist staffer recently – in the end she erred on the side of caution, only to find the mother and sister of one of the grooms both in white lace dresses on the big day). At a swanky black-tie corporate dinner I attended late last year, every male was decked out in a posh penguin suit while the ladies sported everything from a midi-length jumpsuit to a floor-grazing bedazzled white frock that could have moonlighted on Strictly Come Dancing. Put the two women side by side and there was no way you would have guessed they were going to the same event.

One of the problems is that, unlike in Diana’s heyday, our generation finds being overdressed difficult to stomach. Friends and colleagues struggle at the thought of being the only one in a dress in a room full of jeans (although reverse it and they could cope). We’re so used to being able to dress down for work and after-hours drinks (pre-Chanel a/w 2014, would you have worn trainers outside of the gym quite so often?), which has made the more formal dress codes even more of a challenge.

While fashion is one of the most effective ways of communicating our personality, dressing to a code is about belonging. But if you can express your identity within the code boundaries, you’ll feel like you. So when a weighty invitation slips onto your doormat, you should feel excited and elated – not terrified or panicked. And you certainly shouldn’t feel like you’re being coerced into spending a small fortune on a sodding velvet gown because it ticks the ‘black tie’ box. Diana once said she was the only woman in her 20s who wasn’t allowed to dress like one. Yet, while it might have been the case that she didn’t get to have the wardrobe fun and freedom most young women enjoy, she got it right every time.

And so can we all – especially with the help of Stylist’s fashion director Alexandra Fullerton, who’s on hand to translate every dress code you might be faced with…

Diana: Her Fashion Story opens 24 February; from £16.50; Kensington Palace, Kensington Gardens, London, W8; hrp.org.uk/kensington-palace

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• This is the dressiest code for formal events that start after 7pm, such as a ball or gala, so you should opt for a long length. That could be a chic silk dress or a dramatic skirt, which looks ideal with a white cotton shirt and a jewelled necklace.
   
• Don’t be afraid to experiment with a tuxedo. There is nothing chicer when worn with a sheer black blouse and sky-high heels. Look to Evan Rachel Wood’s recent red carpet appearances for inspiration.
   
• Avoid print – it’s too shouty for formal settings. Opt for plain block tones in silk, brocade or Diana’s favoured taffeta, but don’t be afraid of bold colour. Fuchsia is the colour of the season and will ensure your outfit pops in a room full of raven black.

• The default dress code for church weddings or events that start before 6pm, it’s also referred to as ‘formal day dress’. Modesty is key: a long-sleeved midi-dress will be your saviour, styled with point-toe heels for the city or flat sandals for a garden party.
   
• Your top should have thick straps or sheer sleeves and knees should be covered. If you cover your arms, a jacket isn’t required but you may need one to stave off the inclement British summer – double-breasted is ideal.
   
• This is one occasion when a hat is required – never opt for a fascinator. A boater is the most fashionable choice this season. If you’re attending Royal Ascot, measure the base of your hat and width of your shoulder straps to ensure they meet the regulations.

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• Despite the lounging implication, this will be a smart event – a formal dinner, business event or wedding. Men will be in business suits and ties, and women are expected to be similarly polished in a sharply cut cocktail dress or elegant separates.
   
• The holy grail for this dress code is the 18-hour dress – a versatile style that will take you from a full day of meetings to a post-work cocktail with ease. Cleverly chosen accessories will bring the required glamour to your look: a bold cuff, collar-style necklace or embellished shoes are perfect.
   
• If the event is in the country not the city, you can loosen up a bit by choosing a softer silhouette or a floral print – think dressed-up without the formalities of morning dress.

• The most derided of dress codes for its contradictory direction, check the invite for venue information to give you a steer. Use common sense – beach smart casual will call for a completely different outfit to a bar.
   
• The most on-trend interpretations are an embellished sweatshirt and pencil skirt or leather leggings worn with a sharp jacket. Be prepared that some guests might veer more towards the casual element of this dress code, so make sure your outfit looks just as good without a jacket.
   
• Add a metallic flat shoe you can walk in – heels will tip this outfit over into straight ‘smart’ whereas trainers are too informal – and a cross-body bag that carries only the essentials.

Photography: Rex Features, Getty Images, Snowden/Camera Press