"Francis Bacon used the phrase instantia crucis, 'crucial instance,' to refer to something in an experiment that proves one of two hypotheses and disproves the other. Bacon's phrase was based on a sense of the Latin word crux, 'cross,' which had come to mean 'a guidepost that gives directions at a place where one road becomes two,' and hence was suitable for Bacon's metaphor."
– The American Heritage® Dictionary of the English Language, Fourth Edition
Such a cross: St. Andrew's. Some context—
X Marks the Spot scene, "The Last Crusade"
Related symbology for Dan Brown—