“This is how it was when Monsieur Lazzari walked in and found Negus dead,” I said. “Nothing’s been altered in any way.”

Poirot walked over to the open window. “Here is Monsieur Lazzari’s wonderful view that he offered to show me—of the hotel’s gardens. Both Harriet Sippel and Ida Gransbury had rooms on the other side of the hotel, with views of the ‘splendid London.’ Do you see these trees, Catchpool?”

I told him that I did, wondering if he had me down as a colossal idiot. How could I fail to see trees that were directly outside the window?

“Another difference here is the position of the cufflink,” said Poirot. “Did you notice that? In Harriet Sippel’s and Ida Gransbury’s mouths, the cufflink is slightly protruding between the lips. Whereas Richard Negus has the cufflink much farther back, almost at the entrance to the throat.”

I opened my mouth to object, then changed my mind, but it was too late. Poirot had seen the argument in my eyes. “What is it?” he asked.

“I think you’re being a touch pedantic,” I said. “All three victims have monogrammed cufflinks in their mouths—the same initials on each one, PIJ. That’s something they have in common. It isn’t a difference. No matter which of their teeth the cufflink happens to be next to.”

“But it is a very big difference! The lips, the entrance to the throat—these are not the same place, not at all.” Poirot walked over so that he was standing right in front of me. “Catchpool, please remember what I am about to tell you. When three murders are almost identical, the smallest divergent details are of the utmost importance.”

Was I supposed to remember these wise words even if I disagreed with them? Poirot needn’t have worried. I remember nearly every word he has spoken in my presence, and the ones that infuriated me most are the ones I remember best of all.

“All three cufflinks were in the mouths of the victims,” I repeated with determined obstinacy. “That’s good enough for me.”

“This I see,” said Poirot with an air of dejection. “Good enough for you, and good enough also for your hundred people that you might ask, and also, I have no doubt, for your bosses at Scotland Yard. But not good enough for Hercule Poirot!”

I had to remind myself that he was talking about definitions of similarity and difference, and not about me personally.

“What about the open window, when all the windows in the other two rooms are closed?” he asked. “Is that a difference worth noting?”

“It’s unlikely to be relevant,” I said. “Richard Negus might have opened the window himself. There would be no reason for the murderer to close it. You’ve said it often yourself, Poirot—we Englishmen open windows in the dead of winter because we believe it’s good for our character.”