The Y is the municipal device of the City of Chicago. It appears all over the city, mostly on public structures, but often on private property.
It represents Wolf Point, where the Chicago River divides into it's North and South branches, just west of the Loop and Magnificent Mile. Check it out: maps.google.com/maps?f=q&hl=en&geocode=&time=...
TimeOut Chicago says:
Designed by Danish-born A.J. Roewad, the emblem resulted from an 1892 Chicago Tribune contest that sought an image typical of the city in anticipation of the World’s Columbian Exposition. In 1917, the City Council unanimously voted to adopt the Y as the city’s “municipal device.”