וַיְהִ֛י גְּב֥וּל בְּנֵֽי־אֶפְרַ֖יִם לְמִשְׁפְּחֹתָ֑ם וַיְהִ֞י גְּב֤וּל נַחֲלָתָם֙ מִזְרָ֔חָה עַטְר֣וֹת אַדָּ֔ר עַד־בֵּ֥ית חוֹרֹ֖ן עֶלְיֽוֹן׃
Rashi on Joshua
On the east was Atros-addor. The eastern boundary starts from Atros-Addor and from there the border widens to Beis-choron. This is its width.