Use this rule when the first bearing between the object and the bow is 45°, and the second bearing is 90°. Though limited in use, it is useful in certain cases.

 

What it tells you is that the distance run between the two bearings will be equal to the distance to the object when it is abeam.

 

This works because, as you can see in the figure at right, the triangle created when drawn out is isoceles, meaning that two of the sides and two of the interior angles are the same. In a triangle, the sum of the three interior angles will be 180°. If one angle is a, another is 180°-2a, and the last is b, simply solving the equation

180° = a + b + 180° - 2a will show that a = b, and therefore both angles are the same. Because both angles are the same, the sides opposite them will be of equal length.