A design patent protects an article of manufacture. For example, a design patent may cover a chair or even a surgical tool.
Your design must be new and non-obvious to get a patent. New means an identical design did not pre-exist your design. Non-obvious means one or more pre-existing designs do not make your design obvious.
A design patent is limited to the article of manufacture claimed in the patent.
In a recent Federal Circuit decision, the article of manufacture was a "lip implant" that looked like a pencil with a lead tip at opposite ends. The patent office rejected the design application based on an art tool called a "stump" which is used to smooth and blend areas of charcoal. The stump looked very similar if not identical to the lip implant.
The court explained that the stump was an art tool, not a lip implant, and could not be used to reject the lip implant design application.
COMMENT: In the medical device field, for example, many surgical tools have a specific shape to them. This might be suitable for a design patent. Now, objects which are not surgical tools but which have similar shapes to the surgical tool, cannot be used to reject/invalidate your design patent.
This limitation in what is NOT prior art can be used to your advantage. You may find shapes of objects that are of the same type as your design, but still be able to obtain design patent protection.