This is a one piece architrave is also called a pediment or a door header. It is the top horizontal trim piece over the door or window. It will be wider and thicker than the casing. You do not mitre the casing around the door when you use architrave. The casing comes up each side of the door and butts into the bottom of the architrave. Architrave moulding is a very classic look and can come as a one piece mould, or often built up out of three or more separate trim pieces.