Check it out — anyone who has aspirations to be seen wearing the latest look had better get their wardrobe lined up

 

Models present creations by Marc Jacobs' spring-summer parades for Louis Vuitton.

REMEMBER that bit at the end of the film The Da Vinci Code , after Tom Hanks has been looking everywhere for the chalice or inverted pyramid or whatever it is that will lead him to the Holy Grail? After all the deciphering cryptograms and fighting people in churches, he finally realises that what he's looking for must be right back where he started his search, because what matters is the shape, and next to the big glass pyramid at the Louvre there's that little inverted glass pyramid, the one that makes a skylight over the underground gift shops, and so the answer was in plain sight in front of him all along.
Well, fashion for the new season is a bit Da Vinci, except the maths isn't so tricky and hoods are totes over. Just like Hanks, I was trying to figure out what it all means - is the key to a successful new season wardrobe to channel one's inner 1970s groupie (YSL) or planning a look around a surreal fluffy shoe (Celine) or wearing fuchsia top-to-toe (Gucci)? All along, the answer was right in front of me. I was going around in circles, and the answer was a square.
Fashion's holy grail right now is to be square. (Literally. We're talking right angles, not geek chic.) The square is the visual link that connects the season's key fashion shows with the advertising campaigns now launching in the glossies and the It bags now flying out of high-end British department stores. Because it's a shape, rather than a colour, or a decade, or a classic film, it's easy to miss - but once you spot it, the square is everywhere.

Christian Dior's show.
 

Paris fashion week was bookended by catwalk shows designed around squares. It began with Raf Simons' first ready-to-wear show for Dior, which was staged in a giant white cube built for the occasion beside the gold dome of Les Invalides. Inside, the space was divided into a series of interconnecting white rooms, linked by square windows hung with sheer curtains in sugar almond shades. A week later, it ended with Louis Vuitton and a stage set created by French installation artist Daniel Buren, which featured a giant checkerboard catwalk in white and buttercup yellow on which models carried handbags in the Vuitton Damier check. And both Dior and Vuitton have reprised the squares from their catwalk shows to star in their multimillion-pound advertising campaigns, dispelling any notion that the geometry was just a piece of set-dressing whimsy. The Vuitton adverts star models in checks, set against checks; for Dior, the square windows of the catwalk tent become an elegant backdrop for the Dior woman to strike her New Look poses.

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In London, the humble square began to show its style credentials before Christmas, when jumpers with squares on emerged as a surprise competitor to the traditional popularity of the festive jumper. In early December - peak season for a jolly snowflake/reindeer knit - the fashionables were placing orders for Richard Nicoll's latest men's knitwear, which features a simple square of blue or white on a grey background.
Windowpane checks are next up for a revival. They were widely admired on the catwalk for Sportmax in Milan, and feature strongly in the latest Topshop Unique collection.

Models present creations by Marc Jacobs' spring-summer parades for Louis Vuitton.
 

I've got my eye on an extremely nice navy-and-white windowpane skirt-and-tunic co-ord - that's a matching set, do keep up. In my dreams, I'd be accessorising it with this season's best-pedigreed It bag: the Grace Box by Mark Cross, a glorious and defiantly angular mini-trunk, which is a direct descendent of the Mark Cross bag carried by Grace Kelly inRear Window. (Sadly for me, the new bag carries a very 21st-century price tag of almost $2000.)
Like the weather, fashion can only be reliably forecast in fortnightly chunks. But sniff the wind, and you will find that the signs point to squares, straight lines and geometric shapes dominating fashion next season as well as this. Sarah Burton's triumphant spring-summer '13 McQueen collection was themed on womanhood and female power but eschewed curves for the hexagons of the beehive. Alexander Wang, whose aesthetic is clean-lined and graphic (check out this season's knee boots) is about to take over at the storied and influential house of Balenciaga. For now, the learning curve ends here, in straight lines.