Bill found the very reference for me:

The Adventure of the Second Stain, online:

"...The room into which we were shown was that in which the crime had been committed, but no trace of it now remained save an ugly, irregular stain upon the carpet. This carpet was a small square drugget in the centre of the room...

Lestrade: "Well, you know, after a crime of this sort we are very careful to keep things in their position. Nothing has been moved... This carpet...it is not fastened down, only just laid there. We had occasion to raise it. We found...You see that stain on the carpet? Well, a great deal must have soaked through, must it not?"

Holmes: "Undoubtedly it must."

L: "Well, you will be surprised to hear that there is no stain on the white woodwork to correspond."

H: "But the under side is as stained as the upper. It must have left a mark."

L: "Now, I'll show you the explanation. There is a second stain, but it does not correspond with the other. See for yourself." As he spoke he turned over another portion of the carpet, and there, sure enough, was a great crimson spill upon the square white facing of the old-fashioned floor. "What do you make of that, Mr. Holmes?"

H: "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..."

L: "And that means..."

But, dear reader, you must finish it for yourself. "Drugget" appears perhaps half-a-dozen times. The full text is at http://www.asahi-net.or.jp/~KU5M-FJI/ret_sh13.txt
or, of course, in your library.

'..."We also have our diplomatic secrets," said Holmes and, picking up his hat, he turned to the door.'



Thanks again, Bill!