If you’ve never seen a double-casement window, I highly suggest you visit Anlin’s website or call us, because there’s not many of them and it’s a shame. A single casement window is where you crank out the window entirely. A double-casement window is when there’s two frames that need to be cranked out. You can also leave one open, if you wish, but this product gives you the option to have both frames fully open.