Chicago Skyline

Chicago is unusual in that you can read its architectural history straight off the silhouette, because each era left towers standing rather than replacing the last. The 1920s and 30s put up setback towers with ornamented crowns — the Board of Trade with Ceres on top, the Tribune Tower's flying buttresses. The 1950s and 60s brought Mies van der Rohe and the glass box, deliberately plain against all that decoration. Then the 1970s produced the supertalls.

Most cities have one dominant era. Chicago has three sitting side by side, and a model that flattens them into a uniform row of blocks loses the thing that makes the skyline legible.

The Willis Tower is not a tapering tower, it is nine square tubes bundled together, and they stop at different heights. Two end at floor 50, two more at 66, three at 90, and the final two carry on to 108. That bundled-tube structure was the engineering idea that made the height possible, and the stepped profile is a direct expression of it rather than a stylistic choice.

It is also the hardest part to print correctly. Each setback is a sharp horizontal edge, and the twin antennas on top are thin, tall and unsupported — the first thing to break in handling. We print them as part of the mass and keep the proportions honest rather than thickening them until they are safe but wrong.

The model runs the Loop lakefront as it reads from the water. Willis Tower dominates the western end with its stepped bundled tubes and twin antennas. Moving east: 311 South Wacker with its illuminated crown, the Board of Trade closing the end of LaSalle Street, Aon Center as a plain white shaft, and Two Prudential Plaza's chevroned setbacks.

Further north the St. Regis rises in three stacked, curving tiers, and 875 North Michigan — still the John Hancock to everyone who lives here — carries its distinctive tapering X-braced frame. The Wrigley Building's clock tower and the Tribune Tower's crown mark the river mouth. Marina City's two round corn-cob towers are small at this scale and still unmistakable.