“The Red-Headed League” is a quintessential “fair-play” mystery, in which readers know all the relevant clues at the same time that the detective does and therefore should theoretically be able to solve the crime on their own. This type of detective fiction is now common, thanks largely to the popular success of early fair play mysteries such as “The Red-Headed League” and other Sherlock Holmes stories. Because Doyle’s story is such an early example of a fair-play mystery, readers can see Doyle experimenting with this new form of fiction, especially when he has Holmes tell Watson that he should be able to solve the mystery too. Because Watson is a stand-in for readers, Doyle is consequently instructing his audience that they too can piece together the puzzle if they just think things through.

Some readers have complained that Doyle withholds some evidence because Holmes rarely shares his thoughts with Watson. Watson is confused, for example, when Holmes taps the sidewalk outside Wilson’s shop and asks Vincent Spaulding for directions. However, Doyle skirts this criticism by claiming that Holmes often behaves mysteriously because he’s already solved important pieces of the riddle in his head. In this case, he’s already figured out that Spaulding is digging an underground tunnel between the pawnshop and bank and merely wants to confirm his suspicions by listening for hollows under the ground and examining Spaulding’s trousers. Although these actions aren’t necessarily clues, they confirm what readers could have concluded themselves. The influence of fair-play mysteries such as “The Red-Headed League” on detective fiction has been enormous, because many readers enjoy feeling involved in the case, even if the ending still surprises them.