Bo-en-onder deur (stable door)
A door divided horizontally into an upper and lower door, thus allowing the lower half to be closed to keep out animals whilst the upper half can be opened to admit light and air. It also serves to control the draught to the kitchen fire. From about 1815 such doors were frequently replaced by vertically divided paneled doors. Variants included a sash fanlight which could be drawn down to admit light while excluding drafts.
(Walton, 1993: 110 - Walton, 1995: 124)