The scarlet thread

From The Adventures of Sherlock Holmes, illustrated by Sidney Paget, in The Strand Magazine, 1893

"The ring, man, the ring: that was what he came back for.  If we have no other way of catching him, we can always bait our line with the ring.  I shall have him Doctor -- I'll lay you two to one on that.  I must thank you for it all.  I might not have gone but for you, and so have missed the finest study I ever came across: a study in scarlet, eh?  Why shouldn't we use a little art jargon.  There's the scarlet thread of murder running through the colourless skein of life and our duty is to unravel it, and isolate it, and expose every inch of it.  And now for lunch, and then for Norma Neruda.  Her attack and her bowing are splendid.  What's that little thing of Chopin's she plays so magnificently: Tra-la-la-lira-lira-lay."

A Study in Scarlet  [the first appearance of Sherlock Holmes] Arthur Conan-Doyle