A slip dress is one of the most useful things in a wardrobe precisely because it is unfinished. On its own it is an evening dress. With a t shirt underneath it becomes a daytime dress. With a fine knit over the top and the neckline showing it becomes something else again, and with a shirt open over it, something else. The reason it takes layers so well is the cut: bias or straight, no waist seam, minimal shoulder, so nothing interferes with a garment worn over or under it. What matters is the fabric weight. A very light silk or satin will show every seam of whatever is underneath, while a heavier satin, a viscose or a crepe will sit smoothly over a t shirt and hold its own line.

A fine cotton t shirt in a colour close to the dress is the most reliable layer, since a contrasting one turns the outfit into a statement about layering. A long sleeve fine knit under a slip works in cold weather and needs the knit to be genuinely thin at the underarm or the dress will pull. Over the top, a cropped or waist length knit keeps the skirt of the dress visible and preserves the line, while a long cardigan covers the dress almost entirely and changes it into a skirt with a knit. A shirt worn open is the easiest of all. What does not work is a structured blazer over a bias cut, because the shoulder of the jacket and the fluidity of the dress argue with each other.

We think of the slip dress as the highest value garment per pound in a wardrobe, on the condition that it is plain. A print or a heavy embellishment removes the layering option and returns it to being an evening dress. On colour, a mid tone or a deep shade is more useful than black, which reads formal on its own and limits what can go underneath. Straps should be adjustable, since the whole thing depends on the neckline sitting where you want it. If you take one thing away, buy the plainest slip dress you can find in the heaviest fabric you can afford, and it will quietly do the work of four dresses.