Of all the strange outcomes from menswear’s recent decision to abandon pretty much all its rules, one of the most pleasing has been the cross-pollination between styles that never used to speak to each other. The suit has ignored its parents and shacked up with a cardigan, to create tailoring you can just shrug on. Shirts have spent a drunken night with pyjamas, producing boudoir garms you can wear to work. And everywhere you look there’s side-striped trousers, the bastard child of sweatpants and a tuxedo. It’s the logical extension of luxury joggers, this decade’s most illogical trend.

According to Mr Porter editor Adam Welch, there’s two factors at play here: designers’ obsession with the 1970s, which they’ve raided for Tenenbaum-style sportswear; and “vintage militaria”, first seen in Dries Van Noten’s spin on British officers’ red-striped dress trousers, now replicated everywhere (those keen on authenticity can pick up a pair of the army OGs at surplus stores or on eBay for a 10th of the price).

The best thing about side-striped trousers is that, because they draw from the pitch and the parade ground, they work in all manner of ways. “It’s about bridging the gap between sport and tailoring,” says stylist Millie Rich. “So when styling, always go for an opposing top layer.” That means pairing sporty strides with a chunky roll neck, or smart versions with a sweatshirt.

Below, we identify the two key styles to look out for this season and show you how each can hit that up-down sweet spot.

The Track Pant

A riddle for the modern man: when your joggers are so luxurious they’re indistinguishable from formal trousers, are they still joggers at all? We don’t know. But one way to tip the scales is to add the kind of go-faster stripe that’s long made lads lust after Adidas tracksuits.

“The trousers should be the focal part of the look,” says Rich. Bearing the up-down rule in mind, that means avoiding anything too street-influenced elsewhere, especially oversized logos. “Unless you’re going for a very minimal, thin stripe, use the trousers to make a statement and pair with wardrobe staples like a grandad-collar shirt. That way, you won’t look like you’re coming from the gym.”

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The Drawstring Trouser

The tie-up side-stripe sits halfway between two pairs of trousers that are each already halfway between two pairs of trousers (and they say menswear is simple). But they’re less complicated than it sounds: since they bullseye all manner of Venn diagrams, they dress up, down and sideways. “They’re fun, easy to wear and great for adding a touch of character,” says Rich. But don’t be tempted to overdo the detailing. “Stick to one bold colour or a combination of two colours at most.”

For maximum versatility, you need to nail the right combo of form, fit and fabric. Some taper nods to tailoring, but you should have room to move so they still feel sporty. Cozier, weightier materials like wool or denim give the right drape but without shimmer, so you don’t veer into eveningwear. And avoid anything extraneous like pleats or patterns. “You want to ensure the stripe is the boldest part,” says Rich. “Think combos like navy with a red stripe or black with white.”

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