Building a border wall between the U.S. and Mexico has been intertwined with President Trump's political identity since 2015. The wall was a signature campaign promise for Mr. Trump, a slogan and a rallying cry for his supporters. The Trump administration was going to build the wall, Mr. Trump was fond of saying, and Mexico was going to pay for it.

However, in his two years in office, the wall has failed to materialize. Mr. Trump has claimed falsely in rallies that the wall was already being built, and that Congress has already partially funded it. In fact, repairs are continuing on existing pieces of fence on the border, and only the House has passed a bill to fund the wall. Now, Mr. Trump has staked a partial government shutdown on the wall, refusing to sign a spending bill unless wall funding is included.

But the wall is no longer the wall. His description of the wall he imagined when he was a candidate is evolving. In 2015, Mr. Trump said of his wall, "It's going to be made of hardened concrete, and it's going to be made out of rebar, and steel." Since Thursday, Mr. Trump has taken to referring to the proposed structure as "steel slats," and in one tweet, he suggested they would have a certain aesthetic quality.

Get Breaking News Delivered to Your Inbox Click here to view related media. click to expand

"The Democrats, are saying loud and clear that they do not want to build a Concrete Wall - but we are not building a Concrete Wall, we are building artistically designed steel slats, so that you can easily see through it," he tweeted earlier this week.

On Friday, as he talked about the House vote Thursday, he mentioned the slats again as he praised the GOP for "approving strong border security and the money necessary to take care of the barrier wall or steel slats."

He has been consistent about wanting a transparent quality in his southern barrier, though. Speaking to reporters in July 2017, Mr. Trump said that the wall "has to be transparent," in order to see what is happening on the other side.

In this Monday, Nov. 14, 2016, photo, a bird rests on a section of 18-foot high border fence in Brownsville, Texas. The idea of a concrete wall spanning the entire 1,954-mile southwest frontier collides head-on with multiple realities, like a looping Rio Grande, fierce local resistance, and cost. Eric Gay / AP

"As horrible as it sounds, when they throw the large sacks of drugs over, and if you have people on the other side of the wall, you don't see them. They hit you on the head with 60 pounds of stuff? It's over," Mr. Trump said at the time. He doubled down on the need for a transparent wall in a trip to San Diego in March to see prototypes for the wall.

The "steel slats" sound somewhat similar to the fences erected during the President George W. Bush administration. A 2016 Associated Press report described the existing border structures as "fragmented series of fencing, composed of enormous steel bars embedded in concrete close together" which "must reach a height of 18ft loom over the landscape." There are significant gaps between these fences, however.

Mr. Trump has now started referring to the wall and steel slats interchangeably. On Thursday, he tweeted that the "Steel Slats (Wall) are necessary for Border Security." He reiterated this point at a bill signing on Friday.

"Whatever you want to call it, it's all the same," Mr. Trump said in the Oval Office on Friday.

Mr. Trump tweeted a picture of the proposed "Steel Slat Barrier" Friday evening, calling it "totally effective while at the same time beautiful!" The picture appears to be very similar to existing border fencing.