Marvin says its Ultimate Venting Picture Window offers an unobstructed view (no mullions or visible screens) while still allowing airflow. “Opening” the unit involves flipping a pair of handles to push the sash away from the frame enough to create a gap around the edges (a hidden screen keeps out bugs). The window comes in a range of custom sizes; the contractor’s price for a 48-by-48-inch unit at one New England supplier I called was $975. Marvin Windows and Doors, 888/537-7828, marvin.com.