An astragal is a piece of molding used on a pair of doors or between the top and bottom leaves of a Dutch door, to provide security, protect against weather conditions, prevent light or sound transmission, or to retard the passage of smoke, flame, or gases during a fire.  On a Dutch door the astragal is used to close the bottom leaf in conjunction with the top leaf.  An astragal should not be confused with a mullion, which sits between (fixed) or behind (removable) the meeting stiles of a pair.