“Why, it is simple enough. The two stains did correspond, but the carpet has been turned round. As it was square and unfastened it was easily done.”

“The official police don’t need you, Mr. Holmes, to tell them that the carpet must have been turned round. That’s clear enough, for the stains lie above each other⁠—if you lay it over this way. But what I want to know is, who shifted the carpet, and why?”

I could see from Holmes’s rigid face that he was vibrating with inward excitement.

“Look here, Lestrade,” said he, “has that constable in the passage been in charge of the place all the time?”

“Yes, he has.”