This is the first thing I reach for once the heating goes back on properly and the light drops by half past four. A jumper does a particular job in that stretch from October through to March, the weeks when a coat feels like overkill indoors but a t-shirt alone isn't enough, and you want to look like you've made an effort without having actually made one. That's the window it belongs to.
Keep it away from stiff fabrics and sharp tailoring, a jumper wants soft things around it, not a structured blazer fighting for the same space on your shoulders, and definitely not another heavy texture at the same time, a chunky knit over a pleated skirt with a lot going on already just looks like too much wool and too much fuss. Over months, a good one changes in a way that's mostly flattering. The wool softens, the colour mellows slightly, and if it's a decent weight it holds its shape rather than stretching out at the hem and cuffs within three wears, which is the first sign of a disappointing one. Wash it less than you think you need to, cold water, turned inside out, and dry it flat always, never on a hanger, because wet wool over a hanger gives you two long shoulder bumps and a jumper that hangs like a tent. A bobble shaver every few weeks keeps it looking cared for rather than tired.
The difference between a jumper that lasts and one that looks sad by December shows up in a few places. Run your fingers over the knit, if you can see daylight through it when you hold it up, it will pill and thin fast. Check the ribbing at the cuffs and hem, it should have proper spring and bounce back when you stretch it, not stay stretched. Seams at the shoulder and underarm should sit flat, not bunch or twist when you move your arms, and any neckline should keep its shape rather than go wide and gape forward after a few wears, which happens a lot with cheaper knits and is almost impossible to fix once it's started.
Fit is where most people go wrong, usually by sizing up for cosiness and ending up swamped through the shoulder seam, which should sit roughly where your actual shoulder ends, not halfway down your arm. Try it with your arms raised and lowered, the body shouldn't ride up at the front and the sleeves shouldn't climb past your wrist. If it's fitted, check it doesn't pull across the chest when you sit down, and if it's looser, check the shoulder seam again, that's the detail that gives away a bad fit every time.
Worn over the jeans you already wear constantly, it turns a plain pairing into something that actually looks dressed.