The end of summer: the one piece worth buying now

Every year in late August a week catches you out. Mornings are cool. Afternoons warm up. By the time you leave dinner you want something over your shoulders, and none of last winter's coats will do the job.

You feel it first in small things. The bare arms you were happy with last week suddenly want a layer. The linen dress reads slightly wrong at eight in the evening. Your leather jacket, when you pull it out on instinct, feels heavy and too dark for the light that is still in the sky.

This is when a good transitional piece earns its keep. Something between a jacket and an overcoat, in lightweight cotton or unlined wool. A trench in a colour that flatters both a summer dress and a cashmere knit. A long shirt-jacket in linen, or a fluid gabardine coat that skims rather than clings. The right one carries you through six weeks of unpredictable weather.

What separates a good transitional piece from a wardrobe filler is intention in fabric and length. Fabric with structure that still drapes; anything too stiff will feel wrong over a summer dress. A length that either hits mid-thigh or grazes the knee; anything shorter reads jacket, anything longer reads winter. Colours that borrow from the season you are leaving and the one you are entering. Camel, sand, deep cream, oat, a soft ecru.

The best transitional coats do not announce themselves. They earn their place quietly, by being the piece you actually reach for at nine in the evening when the sun is going down and you are still outside. It is the single most useful thing you can buy right now.