We take portable music for granted these days. Any commuter in any big city in the world is more likely than not to have a pair of earbuds or headphones on as they walk, bike, or ride to their destination. The thing is, personal portable music didn't exist for most of human history, at least not in any mainstream fashion. Not until the Sony Walkman came along.

The first of Sony's iconic portable cassette tape players went on sale on this day, July 1st, back in 1979 for $150. As the story goes, Sony co-founder Masaru Ibuka got the wheels turning months before when he asked for a way to listen to opera that was more portable than Sony's existing TC-D5 cassette players. The charge fell to Sony designer Norio Ohga, who built a prototype out of Sony's Pressman cassette recorder in time for Ibuka's next flight.

After a disappointing first month of sales, the Walkman went on to become one of Sony's most successful brands of all time, transitioning formats over the years into CD, Mini-Disc, MP3 and finally, streaming music. Over 400 million Walkman portable music players have been sold, 200 million of them cassette players. Sony retired the classic cassette tape Walkman line in 2010, and was forced to pay a huge settlement to the original inventor of the portable cassette player, Andreas Pavel. But the name lives on today in the form of new MP3 players and Sony's Walkman app. They heyday of the Walkman may be over, with kids today baffled and disgusted by the relative clumsiness of cassettes. But the habit it spawned — listening to music wherever and whenever you want — is bigger than ever.