Some of you may know what these are. Hint: not
Here is what they are:
It is a homemade device that carpenters used to transfer the edge of an underlying board to an overlapping board—marking a perfect cut line.”
Preachers were most often used to mark butt joints in wooden siding and anywhere an underlying line needed to be transfered, such as fitting baseboard. The preacher would straddle the baseboard, tight to the door casing, to mark a cut line using the outside leg of the preacher as a guide, resulting in a more accurate, better fitting joint than if you’d only marked the cut using the visible parts of the casing.