When you edit lines that are nearly horizontal or vertical, commands on the Sketch Relations context toolbar are highlighted to make it easier for you to align the lines. The software evaluates your actions and selectively highlights either the Make Horizontal or Make Vertical command buttons.
For example, if the line you are editing is close to horizontal, the Make Horizontal 
icon is outlined, calling your attention to the command to make it easier to align the line horizontally.
If the line is close to vertical, the Make Vertical 
icon is outlined.
