A city is much the same as a living organism. When overlooking the city from high above, one finds that it is closely linked by crisscrossed roads like numerous blood vessels. Those roads are variously addressed in different times and areas. In Shanghai they are called "Longtang".
It is said that a good understanding of Shanghai should start from Longtang. The appreciation of the legacy of longtang is like reading the history book of the metropolis, which helps us identify her culturul essence. With the speed up of old neighborhood redevelopment projects in recent years, many Longtang have been replaced by modern office edifices and high apartments. Yet Longtang will, as a part of history and a style of culture, linger in the deep of enduring memories of Shanghainese.
The old residential neighborhood in Shanghai is also called "Shikumen", stone framed gate in literal translation, which is now more often used than "Longtang" to represent this traditional style of living organism.
Likewise you will find the similar relation between "Siheyuan" (quadrangle courtyard) and "Hutong" of Beijing. If Longtang or Hutong is described as the the blood vessel, then Siheyuan or Shikumen is the cell.
In the prespective of the architectural genres, Longtang is more cramped hence half open to neighbors next door or opposite as the public facitilies such as bathroom, sink and kitchen are shared by every a couple of adjacent households, while Hutong is composed by individual compound in which houses of one family are enclosed.
So in old times of Shanghai, it is quite usual to pass something to a neighbor without going out of the door. People call this hands-shaking houses, as it were.