How to add elbow to the table
To create an elbow, follow these steps:
1. Type the column heading and row heading in the corner cell.
2. Place the cursor between the two words, press Alt+Enter:
3. Adjust the column heading to the right.
4. Place a diagonal line between the two using the borders option to create a result similar to:
How to add borders, see Apply or remove cell borders.
See also this tip in French: Comment ajouter une ligne diagonale à la cellule.