“I must explain here how I came in possession of this trump card of mine. It was through two casual observations, which at first never struck me as bearing in any way upon the matter I was investigating. The first was the annoying, almost impossible tidiness of Digby-Coates’s hair. It did not appear to be greased or pomaded in any way, and yet I never saw it other than as if he had just brushed it, and with care. The second was his curious trick of sitting on the edge of a chair with his feet thrust first backwards through the gateway formed by the front legs and then outwards until each instep is pressed against the back of each of those front legs. It is a trick most boys have, but it is unusual to find it persisting in a man of middle-age. Digby-Coates does not, of course, always sit like that, but frequently. “What changed these two chance observations—the sort of thing one idly notices about any man of one’s acquaintance without really thinking about them—into perhaps the most important minor step in my case was a glimpse I had of Digby-Coates from the very point from which the servants who made his alibi had seen him. He was sitting as they had seen him sit (though I did not know this until I questioned them) in the big arm-chair, which was facing the window. All I could see from the passage was the long, solid back of the chair, the top of the well-tended head, six inches of each trouser-leg, and the soles of two shoes. On the open door was a notice: ‘Busy—Please do not disturb.’ “The scene was, in fact, a replica of what I had gathered it to be on the night of the murder. I fell to thinking, and suddenly the most annoying pieces of my jig-saw puzzle fell into place. I went in and spoke to him. I looked, more carefully than ever before, at his head, and came to the conclusion that he was bald, but wore the most skilfully made _toupé_ I had ever seen. I remembered that he had told me that he never used a valet. I pictured him—he is the type—as one to whom the thought that any one else knew how unsavoury he appeared minus hair was abhorrent. “When I discovered the _toupé_, I knew that I could smash the alibi if only the unknowing alibi-makers gave me, honestly, the answers I wanted. “As you know, they did. I consider the matter clear, but I know it. Perhaps I had better show what Digby-Coates did that night; how he set his stage and played out his one-act show. “He retires to his room, knowing that Hoode is in his study, Deacon busy or, as often of late, out, Miss Hoode and Mrs. Mainwaring in their beds, and some of the servants, as he wishes them, moving about the house—he has studied their movements and knows that on this night of the week there is work to do which keeps them later than usual. Luckily for him, the night is hot. It gives the necessary excuse for leaving his door as well as his window open. Upon that open door—which is not back against the wall, but only half open—he places a notice: ‘Busy—Please do not disturb.’