butterfly gate
n.— «The insert was among several in the Nov. 15 issue, celebrating the magazine’s 40th anniversary. All the inserts were what the industry calls butterfly gates—ad pages on the outside, which unfold to reveal pages of articles inside.» —“Antismoking Activist Calls Rolling Stone Insert ’One Great Big Cigarette Ad'” by Stuart Elliot New York Times Nov. 26, 2007. (source: Double-Tongued Dictionary)