Extremely practical!
No more risk of getting the wrong direction or angle measurement as with the Cros ruler with its 2 protractors. 1) To read an angle with this ruler, simply put it on the oblique line and turn the wheel until its interior lines are vertical and horizontal, with N (North) at the top! If necessary, drag the ruler a little along the line to make an interior line coincide with a horizontal (latitude) or a vertical (longitude) of the map. 2) Conversely, to draw a course from a point, it is enough (on a white paper for more readability) to adjust the rotating part of the ruler to the right angle, then to apply the ruler to the map with its edge passing through the point (by leaning on a pencil positioned on the point) and by pivoting and sliding the ruler until one of its interior lines coincides with a horizontal or a vertical of the map. This is much faster than with a Cros ruler because it's easier than trying to read the angle while positioning the ruler against the starting point. I strongly recommend for people who have readability problems...