The short answer on Whistles Slim Jersey Jacket Blue: pick one anchor piece, keep the palette to two colours, and let the shoes decide how formal the whole thing reads. Repeat one element, a metal, a texture or a colour, and an outfit looks deliberate. One piece should lead and the rest should stay quiet; two competing pieces read as an accident. Weight matters more than fill number; a heavy coat gets left at home. The groups below run from the decision that matters most to the ones you make afterwards.

What decides this is shell fabric, lining and where the hem lands. Everything else is preference. Two from the first group show the range: ramybrook.com's Delia Tweed Jacket at $395, and jmclaughlin.com's Audrey Jacket at $170. The first is the considered buy; the second is the one to start with. Check the shoulder seam first; a coat that pulls there will never sit right. A coat you cannot layer a knit under is a coat for one month of the year. A higher price mostly buys materials and finishing, and occasionally it buys nothing at all.

Buy the coat for the clothes you own, not the clothes you plan to buy. From the second group, kennethcole.com's Faux Leather Belted Trench ($138) is the one that earns its place, because it works in more than one context without being restyled. The third group is smaller and deliberately narrower, coats and jackets, for the decisions you make after the main one. None of this needs the whole group. One piece from each is usually enough.

There are 20 coats and jackets here from 13 brands, $73 to $1350, all in stock when this was written. We would not buy all of them, and neither should you. Kenneth Cole's Trucker Jacket at $73 is the low-risk way in, and Stephanie Gottlieb's Vault Sale: Confetti Blue Topaz and Peridot Ear Jacket at $1350 only makes sense if this is a category you already use constantly. On whistles slim jersey jacket blue specifically: the difference between the top and the bottom of this list is real but smaller than the price gap suggests. Nothing here is included because it is famous, and price is not used as a proxy for quality anywhere on this page. If you take one thing away: buy the one you can picture using this week, not the one you are saving for later.