A white t-shirt is what they call a “must have”, always. I reckon one ought to have at least 3, in one’s wardrobe, at any time, perhaps more during summer.  So, if you are missing any, here is a great selection from COS:

You absolutely must look at the details on them, their cut and their fabric. Or not, otherwise you will order them all!!

A white t-shirt will tone down a trousers suit:

a skirt look, a little more casual:

Will make your old jeans very chic:

your chinos cool and more elegant:

and your wide leg trousers even chicer:

E.