In 1816, French physician René Laennec listened to a patient's heart through a rolled sheet of paper. As the Science Museum recounts, he wanted an alternative to placing his ear directly on her chest. He developed wooden instruments from that idea: the earliest stethoscopes looked very different from today's.
The doctor listened with one ear against one end of the tube, while the other end touched the patient. This is why these instruments are called monaural stethoscopes. They carried sounds from the heart and breathing through a device placed between patient and listener.
Versions for both ears became widespread later in the nineteenth century. Their shape changed, but the original idea remains recognisable: bringing sounds from inside the body to the clinician's ear.