X marks the spot
This mark shows the location, as in On the postcard, X marks the spot where we picked blueberries. Although the use of a cross or X is probably much older, this term was first recorded in 1813.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.