The Back Door was the secret entrance on the western side of the Lonely Mountain that opened into the passage that led to bottommost cellar of the halls of the dwarves.
The Back Door’s dimensions were described on Thrór's Map as: "Five feet high the door and three may walk abreast." When Gandalf brought out the map at the unexpected party in Bilbo Baggins' home he also produced the small and curious silver key with a long barrel and intricate wards, which would open the door. Later Elrond discovered and read the moon-letters which revealed the method of opening the door: "Stand by the grey stone when the thrush knocks and the setting sun with the last light of Durin's Day will shine upon the key-hole."
When Bilbo and the dwarves reached the Lonely Mountain they eventually found rough steps that led upwards to a little steep-walled bay above a cliff. In the back was a flat wall that rose as smooth and upright as masons' work. Although without any sign of a post or lintel or threshold, nor any sign of bar or bolt or key-hole, they knew that they had found the door. Days passed and then, on Durin's Day, a thrush came to a grey stone in the bay and knocked a snail against the rock. Right at sunset a last gleam of light struck the smooth rock-face and with a crack a flake of rock fell from the wall and revealed a keyhole. After Thorin turned the key in the hole the dwarves pushed the door inward and the way into the mountain was opened.