A shirt worn open over a T-shirt

The outer shirt needs room

An open shirt behaves more like a light jacket than a tucked-in office shirt. It needs enough room to fall beside the T-shirt without gripping the arms or catching across the back. That does not necessarily mean buying a size larger. A relaxed cut in your usual size may provide better proportions than a narrow shirt with longer sleeves.

Try both layers together and reach forward. If the T-shirt sleeves roll into a lump inside the shirt, adjust the sleeve combination rather than simply ignoring it. A close, thin inner layer usually fits more easily beneath a light shirt, while a substantial overshirt can accommodate a heavier T-shirt. The fabrics should move independently instead of clinging together.

Compare the two hems

A shorter T-shirt inside a slightly longer open shirt creates a clear frame. Similar lengths can also work, particularly with a straight shirt hem. A very long T-shirt hanging below a short outer shirt makes a different shape; choose that deliberately rather than discovering it only in the side view. Tucking the inner layer is another way to control the relationship.

A curved shirt tail is not automatically unsuitable, but an extremely long formal tail can look unfinished when left out. Check the back as well as the front. If the shirt covers most of the trouser seat, decide whether that length supports the loose silhouette you want or simply hides the rest of the outfit.

Let colour separate the layers

A white T-shirt under a blue shirt is a straightforward combination, but the inner layer need not always be white. Ecru under olive, grey under navy or black under a pale overshirt can be equally clear. If both pieces are similar in colour, use a visible difference in texture so the open shirt does not disappear into one indistinct block.

Keep the collar sitting naturally and avoid rolling the sleeves so tightly that the forearms feel restricted. Wear the shirt open because you like the resulting shape, not because it cannot close. On a day when the temperature changes, being able to button it comfortably adds usefulness. Finish with trousers that balance the combined volume rather than automatically choosing the narrowest pair you own.