Why are Beijing's lanes called Hutong?

According to historical records, a small scall scale
city first appeared in the Beijing area about 3,000 years ago. It was not until the Jin dynasty in the 12 century that Beijing became the capital city for the first time. At that time, there were no Hutongs in Beijing, just streets and roads. 

The word 'Hutong' is said to originate from the Mongolian language which is one of China's minority language. It means a passage between rows of Siheyuan courtyard house, the traditional residence of Beijingers, each consisting of rectangle courtyard surrounded by one-storeyed tile-roofed houses, usually one to six meters wide, hutongs are where life was going on for the last 700 years since they first appeared in the Yuan Dynasty (1271-1368).