The Prudential Insurance Company is planning to leave the Randolph Street skyscraper it built seven decades ago — and taking the name with it.

The Prudential name has been on the 41-story Loop building since it opened in 1955. The name has also been on the 64-story younger sibling, Prudential Plaza Two or Pru Two, since that building opened in 1990.

The departure is momentous. Prudential stuck with its namesake skyscraper long after Sears, Wrigley, the Chicago Tribune, Montgomery Ward, Kemper and other big corporations left their buildings.

There is more than just a change in name. When Prudential goes, the company leaves behind a lot of the legacy it embedded in that building. The tower was the first high-rise in Chicago built since the Great Depression and the first built on air rights over a vast Illinois Central rail yard that filled what’s now 15 square blocks of high-rises and park land — Illinois Center and Lakeshore East. It helped pave the way for Millennium Park, the Pedway and between 1955 and 1965 was the city’s tallest building.

The Prudential name isn’t just “on the building” — it’s prominently displayed 200 feet long across the top of the older building’s façade in blue letters, and carved into the façade beneath a sculpted rendering of its Rock of Gibraltar logo.

Prudential sold the two buildings in 2000 but has held onto office space and the naming rights. Prudential is planning to move out of its 50,000 square feet in Prudential Plaza and move to a 28,000 square feet space clear across the Loop at 150 N. Riverside Plaza.

Larry Krueger, an executive at the Prudential buildings’ owner, Wanxiang America Real Estate, told real estate website CoStar that “it’s up to us to remove the signs when we find a suitable replacement,” likely by selling the naming rights to a new tenant.

But the Rock of Gibraltar, a 65-ton and 30-feet-high carving on the Prudential Building designed by sculptor Alfonso Iannelli, is “going to stay,” Krueger told CoStar. “We don’t want it to be removed and Prudential isn’t requiring us to remove it.”

In 1949, when New Jersey-based Prudential Insurance Company began looking for a site in Chicago, all the land east of Michigan Avenue and north of the Art Institute was Illinois Central rail yards. It had been that way since the 1850s.

Prudential was decentralizing its business, building new regional headquarters in Los Angeles, Houston, Toronto and Chicago, which would be the biggest.

The negotiations for this site were very secretive, according to Chicago Tribune reports from the early 1950s. Prudential’s real estate representatives had a secret office in the Loop for meetings and a secret phone number, and they wouldn’t even tell the railroad who the potential buyer was. Instead they told district court judge William Campbell, and he told the railroad the client was legitimate.

The deal, when it was finally disclosed, was big news. The last time a tall building had gone up in Chicago was 1934, when the Field Building opened on LaSalle Street a few years into the Great Depression.

Prudential president Carroll Shanks said his company chose “the most dramatic building site in America.” That’s not so much for the view as it is for the nature of the site. Because they were building on the air rights over rail lines, Prudential didn’t buy a single parcel, instead buying hundreds of very small lots, each big enough to hold a caisson and the column on top of it, all placed between rail tracks.

This was not the first tower built over air rights — the Merchandise Mart, the main Post Office and others came earlier — but Prudential’s deal demonstrated, a real estate executive said at the time, that “air rights are now as negotiable as any piece of property” on solid ground.

And while the Pedway had started in 1951 with a tunnel connecting what are now the Red and Blue CTA lines, the Prudential’s connection to the below-ground Randolph Street station (now Millennium Station) was a seed of what would later grow into the network of weather-proof underground walkways.

The architecture firm Naess and Murphy designed the building to take maximum advantage of its south-facing site, putting elevators and other components of the core on the north side so there could be expansive spaces with views down Michigan Avenue and over Grant Park. Unlike Mies van der Rohe, who was building with all-glass walls, Naess and Murphy used traditional window “holes,” in part to keep down the cost of utilities by preventing heat loss through all that glass.

Originally, every window could pivot open to be cleaned from the inside. Each one was framed with a sort of inflatable rubber gasket that could be deflated to pivot and clean the window, then re-inflated to keep it in place. Those windows weren’t replaced by fixed-in-place windows until the late 1990s.

That was during a period of massive transformation of the Prudential building. In 1985, the company announced plans to build the 64-story second building on land it had controlled since the early 1950s. The plans would also tear down a five-story 1960s annex to build the outdoor plaza that now stands in the L shape formed by the old and new buildings. In the late 1990s, Prudential announced plans to sell the building, which led to a $380 million deal in 2000.

In the sale, Prudential kept the building’s naming rights for a decade and later got two extensions. Now all that is over and Prudential plans to move, leaving behind only its giant Rock of Gibraltar sculpture.

It’s not as if they’ll have room for the carving in their new, smaller office space.

Dennis Rodkin is the residential real estate reporter for Crain’s Chicago Business and Reset’s “What’s That Building?” contributor. Follow him @Dennis_Rodkin.

K’Von Jackson is the freelance photojournalist for Reset’s “What’s That Building?” Follow him @true_chicago.