Business casual now covers everything from a suit without a tie to clean denim and a knit. Rather than guess, read the room in three places: what the most senior person in the building wears, what people wear to client meetings, and what people wear on the days nobody external is coming in. Your baseline should sit just above the middle of those three. In practice the reliable formula is a tailored bottom, a jacket that is not a suit jacket, and a top that would look finished without the jacket. That last part is what most people get wrong. If the outfit collapses the moment you take the blazer off at your desk, it is not really an outfit, it is a costume with a load-bearing layer. Trousers in a matte, structured fabric read as more formal than the same shape in jersey.

Three things: fabric weight, hem, and the shoulder. A trouser in a fabric with some body holds its crease and hangs straight; the same cut in thin jersey clings and bags at the knee by lunchtime. Hem length is the cheapest fix available and the one people skip. Trousers should break just above the shoe you actually wear to work, which means getting them shortened for flats if you wear flats. On a jacket, the shoulder seam should sit at the edge of your shoulder, and no tailor can move it cheaply, so it is the one measurement to buy for. Everything else, sleeves and waist and length, is adjustable. Check the lining on a jacket, since an unlined jacket is cooler and drapes softer while a lined one holds a sharper line. And check that you can cross your arms without the back pulling.

Two trousers, one skirt or dress, one jacket, four tops, in a palette of two neutrals plus one colour. That is enough for a fortnight without repeating an outfit, and every piece works with every other. Buy the trousers first and buy two in the same fit if they work, because fit is the hard part and repeating it is not a failure. Keep one jacket that is slightly more formal than everything else for the days that go sideways. A fine knit does more work than a blouse, since it needs no ironing and reads as considered. Steam rather than dry-clean where you can, because dry-cleaning wears fabric faster than wearing it does. And keep a spare pair of flats under the desk.

Buy for the ninety percent of days, not the meeting you have twice a year. Most people build a work wardrobe around an imagined presentation and then wear a knit and trousers every actual Tuesday. Do not spend on a matching suit unless your job genuinely needs one. Do spend on the trousers, get them hemmed, and repeat them shamelessly, because nobody notices repeated trousers and everybody notices a bad hem. If you take one thing away: make the outfit work without the jacket, then treat the jacket as the thing you add when the day gets more serious.