Built in two segments a few years apart, the Monadnock captures the turning point in high-rise construction. Its northern half, designed in 1891 by Burnham & Root, was erected with traditional load-bearing masonry walls (6 feet deep at the base). In 1893 Holabird & Roche designed its southern half, which rose around the soon-to-be-common steel skeleton. The building's stone-and-brick exterior, shockingly unornamented for its time, led one critic to liken it to a chimney. The lobby is equally spartan: lined on either side with windowed shops, it's essentially a corridor, but one well worth traveling. Walk it from end to end and you'll feel like you're stepping back in time.