What it is: This type features two sashes (movable panels) that slide vertically open and closed.
Pro: Opening both the top and the bottom creates a natural convection–cool air comes in at the bottom and warm air escapes out the top. Many models tilt inward so you can scrub the panes from the inside.
Con: The horizontal rail in the middle obstructs your view.
Cost: About $100 to $2,300.