TT #51 · Parashas Va’eschanan · Passuk 6:9
And you shall write them on the doorposts of your house and on your gates.
Torah Temimah points to (1) a house with only one doorpost is exempt, and (2) the mezuzah is fixed on the right side of the entrance (Menachos 34a / Yoma 11b parallels).
English: It was taught: A house that has only one doorpost is exempt from mezuzah, because the verse says “mezuzos” (plural). And it was taught: “your house” — your entry from the right.
Diyuk of the plural “mezuzos” and of “beisecha” as the right-hand entry.
A doorway with only one post is exempt; where there are two posts the mezuzah is fixed on the right side of the entrance.