Which event in the story is a key detail in Holmes's solution of the case?
When Holmes is visiting the neighborhood of Jabez Wilson's pawn shop with Watson, he rings the bell of the shop and John Clay opens the door to invite them in. Holmes merely asks for directions to the Strand. Toward the end of the story, when he is explaining his deductions to Watson over whiskeys and sodas back at Baker Street, he tells his old friend:
I hardly looked at his face. His knees were what I wished to see.
The worn, wrinkled and stained condition of the knees of Clay's trousers proved to Holmes that he was right in suspecting that Wilson had been gotten out of the way so that his assistant could be free for four hours a day to dig a tunnel. When Holmes walked around the corner and saw the branch of the City and Suburban Bank, he was sure the tunnel was being dug for the purpose of looting the bank. The most important event in the story was Holmes's asking for directions to the Strand and seeing the condition of John Clay's trousers. He was already sure that the shop assistant posing as Vincent Spaulding was the master criminal John Clay.