The Jews were commanded to write the divine name on the posts (mezuzoth') of their doors (Deuteronomy 6:9). The Jews, misunderstanding this injunction, adopted the custom of writing on a slip of parchment these verses (Deuteronomy 6:4-9, and 11:13-21), which they enclosed in a reed or cylinder and fixed on the right-hand door-post of every room in the house.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Door-posts'". "Easton's Bible Dictionary".