פּירוש אויף דברים 6:9

מזזות ביתך THE DOORPOSTS OF THY HOUSE — The word is written מזזת, to indicate that it is necessary to fix only one מזוזה to a door (cf. Menachot 34a).

וכתבתם על מזוזות ביתך ובשעריך, “you shall inscribe them on the doorposts of your house and of your gates.” This is so that you will remember the fact that the Lord your G-d is One and unique, both when you enter your house and when you leave it. Rabbi Yitzchok, in commenting on the words: “you shall affix it (some of the blood of the Passover lamb) to the two upright posts and the lintel of your house,” (Exodus 12,7) comments that from here we learn that every time the Torah uses the term מזוזות, it refers to either one of the side posts, except when the word is spelled with a second letter ו. (Talmud tractate Menachot folio 34) He views the two sideposts of the door as a simile for a human being’s two shoulders. The mezuzzah, i.e. the parchment scroll containing the text of the commandment, is to be affixed at the lower end of the upper third of that post, at shoulder height of a person of average height. It is affixed at an angle the upper part of it pointing toward the outside of the house.

מזזות ביתיך ובשעריך, mezzuzot both inside your houses and your gates (courtyards.)

ובשעריך AND UPON THY GATES — the plural is used to include also the gates of courts and the gates of provinces and the gates of cities [in the requirement of mezuzah] (Yoma 11a).