What type of shirt did Lacoste popularize?

Lacoste popularized the polo shirt.

René Lacoste designed a short-sleeved knitted tennis shirt that became the modern polo shirt. He first wore the design at the 1926 U.S. Open, replacing the stiff long-sleeved shirts, ties, and flannel trousers traditionally associated with tennis.

The shirt used breathable piqué cotton, an unstarched flat collar, a buttoned placket, and a longer back hem known as a tennis tail. These details improved movement and comfort: the sleeves stayed in place, the fabric breathed, and the longer rear helped keep the shirt tucked in.

After Lacoste partnered with André Gillier in 1933, the shirt was sold commercially through Chemise Lacoste. It was originally a tennis shirt, but the term “polo shirt” became widely used because the same basic style was adopted for polo and other sports. The crocodile emblem, placed on the left chest, also helped make the shirt instantly identifiable.