The Denim Shirt.
This is a classic and essential piece for summer as well as a good transition piece into Fall. Whether you style it tucked into jeans or worn over a lace dress, the verstitility of this shirt never seems to get old. A personal fave of mine is to wear it with leggings and a long tank with the denim shirt unbuttoned overtop with the sleeves rolled up.
If pairing it with denim, make sure the denim colors don't clash. For example, choose a light colored denim shirt and pair it with darker colored denim bottoms.