For a shape to solve it has to have all its parameters 'defined'. Parameters can get 'defined' by relations but here we discuss how they get 'defined' by edge marking only. Understanding how edge marking 'defines' a parameter will help you understand how many edges need to get marked to solve a shape or to understand why a shape may not be solving with the edge marks it already has.
A shape parameter (such as width) will be 'defined' and its value will be solved if two edges that are perpendicular to the direction of the parameter have been marked. See the figure below:
The width of the box is shown by the red W and the red arrowed dimension line. The edges on the box that are 'W' long are light gray. For the width to solve the two edges that are shown in red need to be marked on a photo and not one of the gray edges. Think of these red edges as cutting and defining the ends of the gray edge so it 'knows' its length. In other words, marking the light gray edges do not help solve for 'w' but marking the red edges does. Of course, the marking of the other edges helps 'define' and solve other parameters.
In the figure below we show a set of minimal edges marked in color that would help solve the three parameters (width, height, depth of the box).
