RR 11: The gilded age and the development of skyscrapers as a result.

Sources:

http://en.wikipedia.org/wiki/Woolworth_Building

http://www.gothereguide.com/wrigley+building+chicago-place/

http://en.wikipedia.org/wiki/Reliance_Building

http://en.wikipedia.org/wiki/Gilded_Age

http://www.thewrigleybuilding.com/history