Parking regulation implemented in
New York City in 1950 and possibly
elsewhere in the world as well to ease the recently
mechanized street cleaning. The idea is this: on a given street running East-West you aren't able to park your car on the
South side of the street for a few hours a day on Mondays, Wednesdays and Fridays. The same is true for the North side of the street on Tuesday, Thursdays and Saturdays.
Nowadays, the frequency has been adjusted slightly depending on a given street's need for cleaning. To my knowledge, nearly all streets in Manhattan still use the Mon/Wed/Fri system, while many in Brooklyn and Queens are only cleaned once a week...
Alternate Side of the Street Parking makes sense, but it makes owning a car in New York City somewhat of a pain because every other day you're forced to hunt down a new parking space...