There's a particular stretch of early fenced weather, somewhere between winter coats coming off and proper summer not quite arriving, when a denim shirt does more work than anything else hanging up. Layered over a jumper on a cold morning, then on its own by lunchtime when the sun's decided to show up. It's the kind of piece that handles a day that can't make its mind up, which is most days really.
If you've never owned one, start with a mid wash in a straight, slightly boxy cut rather than anything fitted. A fitted denim shirt looks brilliant on a hanger and then does something odd the moment you sit down or reach up for a cupboard. The boxier shape gives you room to layer it over a t-shirt now and tuck it into something smarter later, and it won't gape at the buttons the way a snug one will after a few washes once the fabric's relaxed. The difference between a good one and a disappointing one shows up fast in the details nobody photographs: proper metal buttons rather than ones that flake after a hot wash, double stitching along the seams that doesn't fray the first time it catches on a bag zip, and a collar with enough structure to stand up on its own rather than flopping like it's given up. Cheap denim shirts go thin at the elbows within a season. Good ones go soft there instead, which is a completely different thing and worth paying attention to when you're trying one on.
It sits well with almost everything you'd already reach for. Knotted over a plain dress it stops things looking too neat. Worn open over a white t-shirt and jeans it somehow doesn't read as double denim, which still surprises me every time, though I wouldn't push your luck with two matching shades. It's good thrown over leggings on a school run sort of morning, and good again, buttoned properly with the sleeves down, under a blazer for something that needs to look a bit more put together than it actually was to assemble.
For colour, a mid or slightly faded wash is the easiest to live with by a mile. Dark indigo looks sharp for about three wears and then shows every crease and watermark going. A pale, worn in shade hides the general chaos of a day far better and actually looks more deliberate the softer it gets, which is the only time fading has ever worked in my favour. Black denim is lovely for about a month before it turns an uneven grey that no amount of careful washing fixes.
Tomorrow morning mine is going over a plain white top with straight leg jeans and trainers, sleeves rolled, nothing clever about it at all.