In another example of strangers collaborating to share information online, Minnesotastan asked for an explanation for a confusing passage in the novel Free Air:

She rarely lost her way.  She was guided by the friendly trail signs -- those big red R's and L's on fence post and telephone pole, magically telling the way from the Mississippi to the Pacific. (p.69)

At first glance, "L" and "R" would seem to mean "left" and "right", but that wouldn't help someone drive across the country. A commenter knew that "L" stood for the Lincoln Highway. With the help of reddit, we find out that "R" designates the Yellowstone Trail. Why they chose that letter instead of a "Y" is still a mystery. Link