taḥton

תַּחְתּוֹן is an adjective derived from the preposition תַּחַת "under, beneath" by the -ôn ending, exactly parallel to עֶלְיוֹן "upper" and תִּיכוֹן "middle," with which it forms a three-term set. BDB, HALOT and DCH agree on the sense "lower, lowest" and record no second sense; the two sub-entries in STEPBible's lexicon (a common adjective and a "Lower (Beth Horon)" toponymic entry) reflect the tagging of the word inside a place name, not homonymy. Its attested distribution is narrow and wholly concrete: the tripartite side chambers of Solomon's temple (1 Kgs 6:6) and of Ezekiel's temple (Ezek 41:7; 42:5-6), the lower ledge of the altar (Ezek 43:14), the lower pool of Jerusalem (Isa 22:9, over against the upper pool of Isa 7:3; 36:2), the lower pavement and lower gate of the temple courts (Ezek 40:18-19), and Beth-horon the lower (Josh 16:3; 18:13; 1 Kgs 9:17; 1 Chr 7:24; 2 Chr 8:5). The netherworld phrases (אֶרֶץ תַּחְתִּיּוֹת, בּוֹר תַּחְתִּיּוֹת, שְׁאוֹל תַּחְתִּית) belong to the byform תַּחְתִּי, not to this word, a distribution difference worth keeping in view when the two are treated together. Ezek 40:19 and 41:7 carry recognized textual and syntactic difficulties, discussed at length in the critical commentaries on Ezekiel's temple vision, but these concern the architecture described rather than the meaning of the adjective.