Dragon's weak spot

The dragon's weak spot is the single flaw in Smaug's otherwise invulnerable armor, a bare patch on the hollow of his left breast. Its discovery by Bilbo Baggins and its subsequent use by Bard the Bowman form the pivotal mechanism by which the dragon is slain, resolving the central threat of the quest and unleashing the political and military conflicts that follow.

Discovery During the Riddling Conversation

During his second descent into the dragon's lair, Bilbo engages Smaug in a riddling conversation designed to probe the dragon's defenses without revealing his own identity. When Smaug boasts that his armor is like tenfold shields and that no blade can pierce him, Bilbo, feigning ignorance, remarks that he had always understood dragons were softer underneath, especially in the region of the chest. Smaug, absurdly pleased by the flattery, rolls over to display his underside, which is crusted with gems and fragments of gold from long lying on his costly bed. Bilbo, looking closely, sees that there is a large patch in the hollow of his left breast as bare as a snail out of its shell. This observation, made while Smaug preens, is the critical intelligence that Bilbo carries back to the dwarves.

The Bare Patch as a Strategic Asset

Back on the mountain ledge, Bilbo confesses to the dwarves that he has a nasty feeling the dragon guessed too much from his riddles, but Balin comforts him by pointing out that he found out one very useful thing: the bare patch in the old Worm's diamond waistcoat. The dwarves immediately begin discussing dragon-slayings historical, dubious, and mythical, and the various sorts of stabs and jabs and undercuts by which they had been accomplished. The old thrush, who has been listening to all their talk, silently spreads its wings and flies away, carrying the news of the weak spot to the south.

The Thrush's Message and Bard's Shot

The thrush flies to Lake-town, where Smaug is attacking the town in a fury of destruction. It perches on the shoulder of Bard, a descendant of Girion, Lord of Dale, and speaks to him in a language he can understand, telling him of tidings up in the Mountain and of all that it had heard. The thrush instructs Bard to wait for the moon to rise and to look for the hollow of the left breast as the dragon flies and turns above him. Bard draws his great yew bow for the last time, saving a single black arrow that had come from his father and from of old. As the dragon swoops low, his belly glittering white with sparkling fires of gems in the moon, Bard sees the one place where the gems do not shine. The black arrow speeds straight for the hollow by the left breast where the foreleg is flung wide, and it smites and vanishes, barb, shaft and feather. With a shriek that deafens men, fells trees and splits stone, Smaug shoots spouting into the air, turns over, and crashes down from on high in ruin upon the town.

Significance

The dragon's weak spot is the narrative device that transforms Smaug from an invincible force of nature into a vulnerable, killable antagonist. Its discovery depends on Bilbo's observational skill and his ability to manipulate the dragon's vanity through riddling talk, while its exploitation depends on the thrush's role as a messenger and Bard's inherited skill and weapon. The bare patch thus links the hobbit's burglar's cunning, the natural world's intelligence network, and the martial prowess of the line of Girion into a single chain of causation that ends the dragon's reign and opens the treasure of the Mountain to the competing claims of dwarves, elves, and men.