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.

How Princess Diana nailed every dress code