Now every detective knows – indeed it is part of his creed – that, in an affair such as I am describing, nothing is too minute or too trivial to have a bearing upon the case. The old gentleman had been at supper when the crime was committed, and from the fact that the table was only laid for one, I argued that he had not expected a visitor. The murderer could not have been hungry, for the food had not been touched. That the motive was not robbery was also plain from the fact that not a drawer had been opened or a lock forced, while the money in his pocket was still intact. The doctors had certified that the wound could not have been self-inflicted, while there was plenty of evidence to show that there had not been a struggle. From the fact that the front-door was locked, and that the key was in the murdered man’s pocket, it was certain that the assassin must have left the house by the back. There was one question, however, so trivial in itself that one might have been excused for not taking note of it, that attracted my attention. As I have said, the old man had been stabbed from behind, and when he was discovered by the police next day, his overturned chair was lying beside him. This, to my mind, showed that he had been seated with his back to the door when the crime had been perpetrated. When I had examined everything else, I turned my attention to the chair. I did not expect it to tell me anything, yet it was from it that I obtained the clue that was ultimately to lead to the solution of the whole mystery…