A tee and a pair of trousers is a complete outfit. It can also look like you stopped halfway. Stylists have a name for the fix: the third piece. Two basics are the starting point, and one more item on top (an overshirt, a blazer, a cardigan) turns them into a look that seems chosen rather than grabbed.
The overshirt is the easiest third piece to own. It is a shirt cut from heavier fabric, such as flannel, twill, corduroy or wool, made to be worn open over a tee like a light jacket. A shacket is its thicker, more jacket-like cousin. Neither needs a special occasion, and both work on a commute, a weekend or a long travel day.
Three things happen when you add the layer, and you can control each one.
A second vertical line. Worn open, the layer makes two edges that run from collar to hem, with the tee as a stripe between them. The eye follows lines, and a vertical stripe makes the whole outfit look more organised than a single flat block of colour.
A second hem. With only a tee, your outfit has one horizontal edge at the top of the trousers. The layer adds another, and where it ends decides the proportion: a short layer next to a high waistband looks neat and boxy, while a long one reads as a coat.
Texture. Thin cotton next to a woven, brushed or ribbed fabric gives the outfit depth even when every colour is quiet. That is why a check or a corduroy can look rich in a plain outfit: it supplies the interest that colour doesn't.
Wear it open for contrast, buttoned for calm. Open is the default because it gives you the vertical stripe. Buttoned works when you want a cleaner, quieter line, but you lose the layered look. Leaving only the bottom button undone, or the top two, sits in between.
Match the hem to the trousers. A hem near the waistband suits high-rise trousers. A hem near the pocket opening suits most cuts. If the layer is long, wear a straighter trouser and let one of the two shapes stay simple.
Keep the colours in step. The simplest formula is one repeated tone: a layer one step deeper than the tee, or close to the trousers. Add one contrasting colour at most. If the overshirt is a check, pick out one of its colours in the tee or trousers and the pattern will look planned rather than random.
Push the sleeves up. Rolling the cuffs to the forearm and leaving the collar standing makes the shirt look worn on purpose. It also makes a heavier fabric easier to wear indoors.
You may already have a third piece. A flannel or denim shirt, a thin knit cardigan or a blazer you rarely wear will all do the same job over a tee. Second-hand shops are a good place to find heavy shirts with character, and corduroy and wool wear well for years. If you do buy, the picks below show the range from around €30 to a few hundred, but the price changes the fabric more than it changes the idea.