In many camps, Friday's final installation of the spire on top of New York City's One World Trade Center makes it the tallest building in the Western Hemisphere at 1776 feet.
But there's a lively debate as to whether the new building can really claim that title.
The final decision will be made by the Council on Tall Buildings and Urban Habitat, headquartered on the campus of the Illinois Institute of Technology, and according to executive director Anthony Wood, the spire is questionable.
"The feature that counts in the height has to be an integral part of the architecture. The thing that manifests itself is that is it a spire, or is it a piece of technical, functional equipment, like an antenna," Wood said. "The discussion about whether that is a spire or an antennae, is a discussion that is ongoing now, and we're convening our committee to rule on it."
Willis Tower has 108 floors -- four more floors than the WTC, which some people think should count for something.
"You've got to go by more floors," Chicagoan Rob Reiner said. "Otherwise you could just keep putting towers on top of towers -- a big pole."
When Chicago's Sears Tower, as it was called then, was built in 1974, it was the highest building in the entire world, but has since been supplanted by a building in Dubai.
And if the WTC's antenna is ruled out, its ranking will drop all the way back to third place, shorter than Chicago's Trump Tower. In fact, Chicago has four of the top 10 tallest buildings, including the Hancock Building and the Aon Center.
"A lot of media outlets are saying One World Trade Center is the U.S.'s tallest building," Wood said. "I can categorically say that is not correct, because it is not a building until it is finished."
A Willis Tower spokesperson released the following statement on the issue:
Willis Tower is delighted to welcome One World Trade Center to our elite club as one of the world's tallest buildings. It is a wonderful structure, and symbolizes America's resilient spirit. We congratulate the developers and owners. Regarding the issue of the Western Hemisphere's tallest building, we have no comment, as that determination is in the hands of the Council on Tall Buildings and Urban Habitat.
The Council is expected to make a decision later this year.