I was going to save the avenues until after I'd finished with the streets, but I thought it would be fun to do 5th Avenue with all the holiday decorations up, so I decided to give myself a pass. Since 5th Avenue ends at Washington Square Park, I extended the walk to include Thompson St., Church St., and Trinity Pl., which continue the general trajectory of 5th Avenue south of Washington Square. (almost exactly 9 miles). I was joined on this walk by my friend Dave, whose legs hurt.
5th Avenue serves as the east/west dividing line for all streets in Manhattan. It runs from 142nd St., south through Harlem, down the east side of Central Park on the Upper East Side (110th St. - 59th St.), and then south through Midtown / the Flatiron / Greenwich Village to Washington Square Park at 8th St. South of Washington Square Park, Thomas St. picks up the trajectory in SoHo through, Canal St.; and south of Canal St. I shifted east slightly to Church St., which runs to Battery Park on the southern tip of the island (albeit with a couple of name changes).
5th Avenue passes by too many landmarks to list, so it might be best to discuss the two things it's best known for: 1.) Expensive housing; 2.) Expensive shopping.
Expensive Housing: While there are expensive apartments and mansions up and down 5th Avenue (and the rest of the city, for that matter), the largest and most notable concentration exists on the east side of Central Park from 110th St. to 59th St. At one time mansions lined a much more substantial stretch of 5th Avenue, but many were demolished to make room for skyscrapers as the commercial center of the city moved north to Midtown. Many of the mansions on the Upper East Side stretch of 5th Avenue suffered a similar fate, but were replaced by grand apartment buildings instead of commercial buildings. The trend toward apartments and away from mansions on the Upper East Side began in earnest in 1916, when a mansion at 72nd St. was torn down and replaced by a 12 story, 24 unit apartment building. The ensuing boom in apartment construction angered many of the 5th Avenue mansion owners, who felt that the new buildings were too tall and clashed with the style of the neighborhood. In 1922 they successfully lobbied the city into restricting the height of new apartments along 5th Avenue to 75 ft. (approx. 5 stories), but the law was struck down a year later, paving the way for further apartment construction. Today, this stretch of 5th Avenue is lined with limestone apartment buildings and only a few mansions remain, all (or almost all) of which have been repurposed as museums, consulates, etc.
Expensive Shopping: Fifth Avenue is home to many of America's most notable luxury stores, including Lord & Taylor, Saks Fifth Avenue and Bergdorf Goodman. According to Wikipedia, the stretch of 5th Avenue running from 47th St. - 59th St. is consistently ranked among the most expensive shopping street in the world. I don't know if that's based on how much the merchandise costs or how much the real estate costs, but it could easily be both. More or less, if you can think of a luxury brand (domestic or foreign), it probably has a flagship store somewhere along this stretch of Fifth Ave.