Jersey trousers suit anyone who wants to look dressed without feeling dressed, which is most of us most days. They're brilliant for a body that wants to sit, bend, drive, and not negotiate with its own waistband. If you need a sharp line and a bit of structure to feel like yourself, though, think twice, because these will never give you that. They move with you rather than holding anything in place, and on a day when you want armour, that softness can feel like the trousers have given up before you have.
The good jersey ones keep their shape for a surprisingly long time, but you can tell within a minute of trying them on which ones will and which ones won't. Pull the fabric across your knuckles and let go. If it springs back flat with no shadow of a crease, that's a decent weight with proper recovery. If it stays stretched and looks tired before you've even left the changing room, that's what your trousers will look like by lunchtime every day you wear them, forever. The ones that pass that test tend to hold their shape through a lot of sitting down, washing machines, and being balled up in a bag at the bottom of a wardrobe for a weekend away. Wash them inside out, on a cool cycle, and resist the tumble dryer, because heat is what kills the stretch and turns a good pair baggy at the knee within a couple of months. Left to dry flat or on a hanger, a decent pair will see you through most of a year without losing its nerve. What they don't love is anything that fights their softness: a stiff structured blazer on top just highlights how unstructured the bottom half is, and a chunky belt usually just folds the waistband rather than defining it. They're much happier with something soft or fluid up top, so the whole outfit reads as deliberate rather than like you gave up getting dressed halfway through.
Where they really come into their own is anywhere you're on your feet, sitting for long stretches, or travelling, and in the colder months when you want something warmer than cotton but you're not reaching for wool. They're less convincing somewhere formal with a dress code, or in high summer, when the weight that makes them comfortable in October makes them a bit much in July. I'll admit I resisted jersey trousers for years, convinced they'd always look like loungewear that had wandered out of the house by mistake, which says more about the bad ones I tried first than the good ones that exist now. A well chosen pair quietly proves that wrong.