You may have heard the terms XO or OX used when talking to window and door companies. What this is determining is the slide direction of doors and windows. X is always the sliding panel (vent) of a door or window. O is always the fixed or non-operable part of a door or window. Always looking at your home from the outside, an XO sliding patio door has the sliding panel (vent) on the left side and the O or fixed panel on the right. Just remember that you are always on the outside looking towards your home.