A design patent protects only the physical appearance of your invention. As such, the drawings in a design patent are the cornerstone of the application and should provide the Examiner with enough views of the object to recreate the physical shape of the object. Every line in a design patent is important, and the breadth of the application can be controlled by choosing whether to make lines solid or broken (usually dashed or dotted).
Broken lines represent unclaimed material that is intended to aid in interpretation of the physical appearance of the claimed object without limiting it. As such, broken lines can enhance an Examiner or layperson’s understanding of the object’s physical appearance but do not narrow your protection.
Solid lines represent claimed features of the design patent, so should be chosen with care and reserved for features that are critical to the design.
Nevertheless it is best to use broken lines for as many aspects of the invention as possible without reading too much on the prior art. During examination, changes that narrow the scope of the patent application are allowed, but it is impossible to broaden an application; amendments broadening the patent by converting solid lines to broken lines will be rejected except in exceptional cases.
In some cases, in fact, an Examiner will strongly suggest that you change certain lines that are indefinite to broken lines. A solid line is judged to be indefinite if its contribution to the contour, shape, or connection to the rest of the object cannot be determined from any of the drawings. In certain cases, this can seriously undermine the protection of the patent application if the solid line is integral to the object (for example, the contour lines that illustrate the unique shape of a bottle). But on the other hand, since the solid line is no longer claimed, the application is broadened. In cases where the solid line was not crucial to the application, this may actually improve the value of the patent.
The point is, you don’t want a narrow patent – what if a competitor places a feature in a different orientation, or doesn’t put one at all? What if he provides a different implementation that serves the same purpose? You wouldn’t be protected in those cases if you made the feature solid. Thus, it is recommended to make a line broken wherever appropriate.