In 1816, René Laennec, a French physician, was visited by an overweight young woman with heart issues. He was having trouble diagnosing her using his hand to feel her heartbeat. Because of her age and sex, he did not want to place his ear against her chest.
Knowing that putting an ear to an object allows the ear to hear noises affecting the other side, he rolled up a sheet of paper into a tube and pressed it against the patient's heart. As he suspected, it allowed him to hear her heart much better. After years of development, the modern stethoscope was finally invented in 1851.