Why is the word "Mezuzos" written without the first 'Zayin'?
Rashi and Targum Yonasan: To teach us that one needs to place a Mezuzah only on one of the doorposts. 1
Targum Yonasan On the right-hand one (as the Gemara learns from the word "Beisecha"
And what are the implications of the word "u'vi'She'arecha"?