The sweater slung over the shoulders, sleeves loosely crossed, was already standard preppy dress in the 1960s, and Lisa Birnbach's Official Preppy Handbook turned that whole look into a bestselling joke in 1980. Inspired by that book, by a September afternoon that starts warm and ends cool, and by a knit that is always there when the wind picks up, this is how I wear the knot now. Pick a lighter knit in a clear color, like Tibi's blue silk mohair crew or Anine Bing's ecru Tiana, because it drapes without bulk.

The knot looks best over something very plain, so the sweater is the only color. A white tank is the base: Anine Bing's Sixtine, Tibi's fine rib tank or Isabel Marant's Thea. For a dressier version, swap jeans for a slip skirt. Brunello Cucinelli's brown satin maxi or Tibi's black Tech Crush skirt, with a white tank and a navy cardigan from The Park over the shoulders, is a whole outfit for dinner. Tie the sleeves low on the chest rather than at the throat.

It works just as well with a dress. Over a slip dress or a simple summer dress, tie the sleeves loosely at the chest so the sweater sits on your shoulders like a small cape, and keep the dress one plain color so the knot is still the accent. On a cool evening, it is the difference between a sundress and an outfit.

For weekends it comes down to jeans, a tank and a brown leather tote. A straight or relaxed jean, like DÔEN's Paulene or Anine Bing's Hugh, keeps the proportions easy, and a roomy brown tote, like DeMellier's New York or The Row's Parker, sits under the arm without crushing the knot. What I would skip: chunky cable knits, which drag the neckline down, and a heavy crossbody strap that fights with the sleeves. If you want to keep browsing, I also put together Wrap Sweater and Sweater Dress.