Extend String
To extend the string and restrict the new point to the direction of the last segment, hold down the CTRL key as you digitise the point with the mouse.
To extend a string and restrict the new point to be perpendicular to the last segment, hold down the SHIFT key as you use the mouse to digitise the point.
To extend the string and restrict the point to the screen axis, hold down the x, y, z, u, or v letter key as you digitise the point with the mouse.
Use the Extend String to Polyline tool on the String Editor Tools toolbar to extend the strings you select to a nominated boundary string.