As the government shutdown continues in the absence of a compromise between the Trump administration and Congressional Democrats, the possibility President Donald Trump will declare a national emergency in order to build the border wall looms ever nearer. If indeed that is what happens, what sort of wall would President Trump prefer? And which companies would be the more likely candidates to construct it?

First some background: The U.S.-Mexican border is almost 2,000 miles long. Out of those, about 700 miles are currently fenced or barricaded to deny entry. The rest of the 1,300 miles are “safeguarded” by natural barriers, such as the Rio Grande River or mountainous terrain.

According to U.S. Customs and Border Protection, a border wall would have two main objectives: to impede and deny. “A major factor in determining where investments in impedance and denial would be most effective is referred to as ‘vanishing time,’” states the agency’s website, “which is the distance between the border and the point at which an illegal border crosser could blend into the local populace. Vanishing times are often particularly short in urban areas.”

President Trump’s statements suggest that he might favor a steel border wall in an attempt to boost the U.S. steel industry, thus hitting two birds with one stone. In a statement last week, he said, “I’ll have it done by the United States Steel Corp., by companies in our country that are now powerful, great companies again. And they’ve become powerful over the last two years because of me and because of our trade policies.”