Story highlights The border wall remains one of Trump's most enduring campaign promises

Trump also scaled back significantly the number of miles he predicts

Washington (CNN) President Donald Trump's long-promised border wall will have "openings" to see through, he said Wednesday night, acknowledging what his Homeland Security team has said is a necessary design feature of any such barrier.

Trump also scaled back significantly the number of miles he predicts such a wall will span, to at most half of the southern border.

Speaking with reporters on Air Force One on the way to Paris, Trump described what he still wants for his pledged border wall -- which thus far Congress has not appropriated any money to build.

Picking up on the theme laid out by Homeland Security Secretary John Kelly and other border experts and agents, Trump spoke about the need for any border barrier to allow agents to see the other side. Much of the current barriers along the border are metal fencing.

"One of the things with the wall is you need transparency," Trump said.

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