Tronera: Opening between three butts, one on top of the other two. An opening between butts in an andana.// Mar. Each of the openings made in the parapets and other parts of a corsair or merchant ship for firing through at the enemy as they try to board.// Acad. Opening in the side of a ship, the parapet of a wall, or an artillery entrenchment, from which guns can be fired safely and accurately.