Black Gates

In the northwestern corner of Mordor, where the western mountains of Ephel Dúath met the northern range of Ered Lithui, they formed a narrow pass that led into Sauron's realm. There the Dark Lord erected a vast stone arch across the pass, and beneath it two immense doors of iron to guard his frontier. This was the Morannon, most commonly called simply the Black Gate, but occasionally the plural form 'Black Gates' was used in reference to the two doors that made up the full gate.

In the first edition of The Lord of the Rings, the Morannon is described at one point as being constructed of three separate gateways, and this is probably the textual origin of the phrase 'Black Gates'. In all later editions, that reference to three gates was amended to a single gateway with two doors (to avoid inconsistency, since the Morannon was elsewhere referred to as a single gate). The plural term 'Black Gates' was retained in a few locations, but following this amendment the 'gates' in question must necessarily have been its two iron doors, rather than the original three separate gateways.