September is the most awkward month for getting dressed. It is fifteen degrees in the morning, twenty-eight in the afternoon, and by evening the jackets come out again. This is the season when people either change three times a day, or give up and put on the wrong thing.

The answer is an item everyone already has in the wardrobe and nobody counts as clothing - the white T-shirt. Not as a fallback when there is nothing else, but as the base on which everything else is built. Fashion desks devoted entire outfit lists to it this year, which is in itself an admission that fashion runs in circles and that this particular circle is comfortable.

The logic is simple: the T-shirt is not what you see, what you see is everything around it. With jeans and a blazer you get something that works in the office on Friday and over coffee on Saturday. With a grey mini skirt and a beige cardigan you get the style that stylists call "quiet luxury" - which translates as clothing that does not want to tell you what it costs.

The most interesting combinations are the ones where the T-shirt works as contrast. With a satin slip skirt - underwear that became outerwear, a trend running for several seasons now. With tailored bermudas and a trench coat, which extends the bermudas by another month instead of returning them to the box. With a long dark floral skirt with ruffles, cinched at the waist, where all the white T-shirt does is calm the whole picture down.

The details carry the difference. Mesh ballet flats in strong colours, which have joined the category of shoes described as "odd" right up until everyone starts wearing them. Stacked bracelets over the sleeve of a long-sleeved tee. A studded belt over plain capri trousers. These are moves that cost little and show a lot - which is exactly the opposite of what the industry would like you to buy.

There is one quiet truth in the whole story. Outfit lists exist to show you that you do not have enough, but read them carefully and most of them start with things you already own: jeans, a blazer, a bag, sunglasses. If you can put together eight of the ten looks at home tonight, then the question is not what you are missing, but why you think you are missing anything.