Headphones were first developed in the late 19th century. Initially, they were created for telephone switchboard operators to keep their hands free. The earliest versions had only one earpiece and were mounted on the head by a clamp to hold the telephone receiver next to the ear. By the 1890s, a listening device with two earpieces was developed by a British company called Electrophone, allowing users to listen to live performances via telephone lines. French engineer Ernest Mercadier patented a set of in-ear headphones in 1891. The modern style of headphones, with two padded ear cups connected by a headband, was invented by American engineer Nathaniel Baldwin in 1910. He made the first pair by hand in his kitchen, and the U.S. Navy placed a large order due to the design’s effectiveness without needing an external power source. Further developments included the first stereo headphones by John C. Koss in 1958, who designed headphones specifically for music listening. The introduction of portable headphones became significant with Sony's Walkman in 1979, which popularized portable music listening.