Magusa (秣) – a horizontal member that forms the top frame of an opening — whether it be for a window, door, or simply a framed, doorless entrance. Often, an opening does not extend the full vertical height of the wall — for example, a window — and therefore needs framework for support. Magusa is the top horizontal member of this support framework.
This particular magusa is from a 2 by 4 house and is reinforced heavily due to it needing to support the spanned distance of the entrance. Conventional houses use heavy beams to support the second floor and so don’t require the magusa to support anything other than the door or window itself.