Ever since Mies Van Der Rohe's proposed glass skyscraper for Berlin, modern architecture has been working toward the vision of the tall, skinny, glass box.
A few modern architects tried for that vision, and we have Lever House and the Seagram buildings in New York by Gordon Bunschaft and Mies himself, respectively, as well as the Johnson Wax Research Tower by Frank Lloyd Wright, but nobody got that glass wall down to almost invisible, the Holy Grail of modernism. There was always too much mullion, or too much structure showing.
In 1972, Pei finally managed to pull it off with the Hancock Tower in Boston.
Pei wrapped up the modern movement and tied it up with a bow in that building. Each bay of each floor is a single pane of glass; there are no spandrels between the floors and the mullions are minimal. In a stroke of genius, Pei took the rectangular floor plan and made it into a parallelogram so that from the most common views the corners of the tower were even sharper. You expect the corners of a skyscraper to be square and those acute angles make it seem like it's standing on point shoes. The reflective glass is tinted blue, so on a bright day it's just a bit of transplanted sky: a giant slab of Magritte in the Boston skyline. For a final touch he added a pin-stripe of geometry to the shorter sides so they would always reflect a different piece of the sky, emphasizing the vertical lines.
As far as engineering goes, the building was a nightmare. Many people know about how the windows started blowing out shortly after it was built, something that nearly ended Pei's career. Inventing a way to use that lovely blue mirror glass in a steel tower came at a high price. But even that's relatively minor: one entire floor is dedicated to two large metal counterweights, added after the fact, that counterbalance harmonic vibrations in high winds. Least known and most surprising is that much structural steel had to be added to the central core because it was possible (unlikely, but possible) that just the right wind condition it could be blown over, not broadside, but on its short side. And there are sump-pumps in the basement that constantly pump water away from its foundation. They pump it back into the ground a few feet away, since Trinity Church, just across the street, was built on wooden pilings that can't be allowed to dry out because if they did they'd begin to rot. Sometimes you have to take extreme measures to get a building that looks just so. I think it was worth it.
There really wasn't anything left for modern architects to do with skyscrapers
after that, and the profession moved on to other things. An entire school
of architectural theory was presented with the ultimate example of its
craft. Architects turned to post-modernism and began to re-examine ornament.
Ornament is back in now. Form doesn't necessarily follow function anymore.
A chapter of architectural theory was closed with the ultimate building
of its style.