Q. Why do we put a mezuzah on a slant?
A. There was a difference of opinion amongst the sages as to whether the mezuzah should be placed in a vertical or perpendicular position. Adopting a slanting position with the top facing inwards was a compromise.
The rule therefore is that a mezuzah is affixed to the right-hand doorpost of the front door and every room except the bathroom and toilet, within the top third of the doorpost, and with the top slanting inwards.
Some discern in this law a principle of human conduct. Everywhere in society, but especially at home, human relationships need to be built upon give and take – i.e. compromise. Adamantly insisting that you – and only you – are right makes life impossible. Entering a room, a person is reminded of how to be a mensch by looking at the mezuzah.