Possible Clues:

• Slanting, from corner to corner
• Line from one corner to the fartherst one
• A straight line joining two opposite corners
• Line connecting two nonadjacent corners of a rectangle
• Slanting line joining two opposite corners
• Line from corner to corner
• Corner-to-corner line
• Sloping line
• Like this line: /
• Straight line joining any two non-adjacent corners of a polygon