I see the X marks the spot at the beginning of a book, marking the start of the book's journey, the 'thing' that catapults readers into the book's action. To me, this spot is not an end point; therefore, it's completely different than 'X marks the spot' on a treasure map.
Think of the book Wonder by R.J. Palacio, the X marks the spot would be when Auggie goes to school. Or consider Okay for Now by Gary D. Schmidt, the X marks the spot is when Doug's father declares the family must move. In The Emerald Atlas by John Stephens, the X marks the spot is the three siblings going to Dr. Stanilaus Pym's orphanage.
But how easily can we place an X marks the spot in our own writing? So that's my question for you today--can you mark that spot in your own writing, that point where everything is different because of one thing? Or, if you don't write, can you X marks the spot easily in the books you are reading? Because whether writing or reading, the X marks the spot should be easy to spot.