The reason it came to be called Hamaguri (Clam) Gate was that during the great Tenmei fire (on January 30, 1788 in the old calendar), the gate, which had until then always remained closed, was opened for the first time, and thus resembled a clam's shell opening in response to being subjected to fire.
蛤御門の名前の由来は、天明の大火（1788年1月30日(旧暦)）の際、それまで閉じられていた門が初めて開門されたので、焼けて口を開ける蛤に例えられた為である。