Patent practitioners and clients alike are understandably nervous about negative limitations in claims. Yet these do have usefulness, especially where there is close art. Negative limitations can properly be used to exclude what is shown in a known reference. Of course, support in the specification is required.
Consider a claim to a combination of A and B. The inventor has figured out a new and useful device, or perhaps a method, that applies this combination, and does so without use of element C. During prosecution of the patent application, the Examiner finds a reference with A, B and C, and issues a rejection of the claims as being anticipated by or unobvious over this reference. What do we do? The rejection appears proper, since the combination of A, B and C includes the sub combination of A and B.
One strategy would be to find (in the present specification) an element D that is not shown in the cited reference, and amend the claims to recite the combination of A, B and D. Of course, this is narrower than the desired combination of A and B, but this might be an acceptable claim.
Another strategy, using negative limitations, is to amend the claims to recite the combination of A and B, free from C. Alternatively, amend the claims to recite the combination of A and B without C, or the combination of A and B excluding C. The patent practitioner can develop other such exclusionary language, and if there are multiple independent claims, it would be wise to use differing language in each of the claims. Arguments accompanying the claim amendments should emphasize that the cited reference requires the use of element C, and that it is therefore unobvious to use the claimed combination that specifically excludes C.
When can this be troublesome? Negative limitations can be overbroad in what is excluded. Negative limitations can be found unsupported in the specification. Negative limitations can be ruled indefinite, if there is insufficient structure (in a structural claim) or unclear steps (in a method claim), and the claim is thus found to attempt to describe a structure or a method only by what it isn’t. One test to apply, when drafting negative limitations, is: are the boundaries of the claim clear?
Here is a particularly subtle effect of negative limitations. Suppose the originally filed claim set had the independent claim of the combination of A and B, with a dependent claim that adds C to this combination. So far, so good. The dependent claim is narrower than the independent claim, and both the independent claim and the dependent claim are therefore proper. But, if we amend the independent claim to exclude C, suddenly the dependent claim becomes broader than the independent claim. Oops! The dependent claim may well be rejected as indefinite. We may have to cancel the dependent claim, which the Examiner likely would have ruled anticipated by or obvious in light of the hypothetical reference described above. On the other hand, in this case, if the inventor does wish to pursue a claim that has A, B and C, then adding D to the independent claim might be a good strategy. An even better strategy might be to pursue both courses of action, using negative limitations to exclude C in one of the independent claims, and reciting A, B and D in another independent claim. The first independent claim could have a dependent claim adding D, and the second independent claim could have a dependent claim adding C. This would give broad coverage.
In order to be successful with negative limitations in claims, it is well to write a specification that describes not only what is used in the invention, but how the invention allows embodiments that do not require the use of C. It is well to write claims that narrowly and specifically exclude only in accordance with what is supported in the specification. When writing arguments, the patent practitioner should exercise caution as these arguments are then in the prosecution history. Where possible, a parallel strategy of having at least one independent claim that doesn’t have the negative limitations should be included. The interested reader is encouraged to read up on negative limitations in the Manual of Patent Examining Procedure (MPEP), which cites many useful cases for further study. This is all part of the art of patenting.