https://en.wikipedia.org/wiki/Mezuzah

It's Wikipedia, so take it with a grain of salt. But this is what I remember an Orthodox Jew telling me.

Edit: I was wrong about which side of the door it's affixed to.

https://en.wikipedia.org/wiki/Mezuzah

It's Wikipedia, so take it with a grain of salt. But this is what I remember an Orthodox Jew telling me