Negative Limitations in Patent Claims – Are They All Bad?
The conventional wisdom is that, in claim drafting, negative limitations should be avoided. Where possible, it is better to express a limitation in positive terms than negative. This is generally considered best practice.
But, it is interesting to consider why this is so, what the hazards are, and whether negative limitations are indeed particularly prone to these or whether they apply across the board.
The conventional viewpoint tends to be that negative limitations – that is to say, claim integers reciting the absence of some feature – are ill-advised as they are more likely to be open-ended than their positive counterparts and may fall foul of various requirements of patent law.
The main stumbling-block would appear to be support. A negative limitation (“an A configured without an X”) is often likely to define an indeterminate or at least “elusive” category of things (all As configured without Xs). While there may be verbatim support for the term “without an X”, the specification is unlikely to contain disclosure of all possible configurations of “Xless As” – all ways of configuring an A without an X. As such, it might be alleged that this claim term is not supported across its full scope by the disclosure.
In contrast, for a positive limitation – “an A having a Y” – a good drafter will have described at least one exemplary Y (and its applicable variations) in the specification to an extent sufficient to point a skilled addressee (otherwise known as a “person skilled in the art” (abbreviated to PSA or, in the US, PHOSITA)) in the right direction.
So, it would seem true that positive limitations are safer at least in the sense that features which “exist” are more readily describable than features which don’t, making it easier to fulfil disclosure requirements. One might say that negative limitations are apt to “leave more to the imagination”, which in patent drafting is not a good omen.
In saying that, there are ways of drafting positive limitations that are famously unspecific – functional language (“configured to [enable / actuate]”, “configured for use with”) being the obvious candidate. Such phrases may likewise define a category that may be indeterminate or “elusive”, and one which in all likelihood will not have been comprehensively traversed in the description. This would tend to suggest that, depending on their drafting, positive limitations might be as vulnerable to support issues as negative limitations.
The case of Ronneby Road Pty Ltd v ESCO Corporation 2015 APO 3 1 dealt with negative limitations. At issue were the integers “free of threaded adjustment” and “moveable without a hammer” (the latter apparently being a reference to a term of art). The Delegate stated that “there is nothing impermissible in having a negative limitation in a claim, provided its meaning is clear”; and found both of the contested limitations to be “perfectly clear” and “supported by statements in the description” 2.
As far as support goes, it appears that the Delegate’s focus in that case was firmly on verbatim support. Paragraphs  and  of the specification indeed stated that the system is “easily movable without reliance on threaded members” and that the lock is “operable without a hammer”. However, these were the only explicit references to those features in the specification, which went on to describe an intricate arrangement involving a lock with resilient portions as well as a hinge portion and “catch” portions, fitting into complementary recesses on other components. On the whole it would seem that what was substantively supported vis-a-vis the lock was not too much broader than the specific embodiment actually described. Yet, even accounting for “hammer[less]” being a term of art, both “free of threaded adjustment” and “moveable without a hammer” would seem to be broad phrases which could be satisfied by a relatively wide array of different configurations.
It is not advisable to assume that verbatim support will always suffice. On the contrary, claims should be “stress-tested” during drafting by asking whether each feature is supported across its notional scope by the disclosure. This doesn’t mean that every single possible embodiment must be described in detail. Rather, sufficient technical “pointers” should be given to allow the PSA to envisage how that feature might be embodied across its scope.
The EPO’s G 1/16 decision 3 (and related decisions) would tend to confirm the prominence of the “support” criterion when it comes to negative limitations; couching as it did the issue of negative disclaimers as a question primarily of support (added matter) per EPC Art. 123(2) 4.
The USPTO’s guidance on negative limitations 5 states that “there is nothing inherently ambiguous or uncertain about a negative limitation” and notes that “[s]o long as the boundaries of the patent protection sought are set forth definitely, albeit negatively, the claim [is compliant]” but that “[a]ny claim containing a negative limitation which does not have basis in the original disclosure should be rejected”. (Other jurisdictions not covered in this article may, of course, have their own rules relating to negative limitations).
Scope / clarity
In addition to “support”, we also considered whether negative limitations might be more vulnerable to “scope” and / or “clarity” issues, in the sense that the bounds of a negative limitation might tend to be somehow more “hazy” than a corresponding positive limitation.
However, at least on one school of thought “an A without an X” is just as definite in its scope as “an A having a Y”. Just as the latter will encompass all As having a Y (but not As without a Y), the former will encompass all As without an X (but not As having an X). Both are equally “binary” and as such their boundaries are equally clearly demarcated. Of course, this will depend on the particulars of the integer in question. But, generally speaking, it is not scope per se, but rather support across the full breadth of that scope (which may be vast, owing to the nature of negative limitations), that is likely to present the principal difficulty for negative limitations.
The main “hazard” with negative limitations would seem to be lack of support (in the substantive sense). Comprehensively describing a “positive” (present) feature is, in practice, more doable than describing a “negative” (absent or non-existent) one. Thus, generally speaking, for positive limitations the description may be more likely to provide adequate support across the full scope of the integer in question.
That said, there are plenty of “open-ended” ways of drafting positive claim limitations, most notably functional claiming which has the advantage of versatility but the corresponding disadvantage of non-specificity. Such drafting devices may likewise be vulnerable to support (and other) issues. So, it not necessarily clear that negative limitations are more vulnerable to objections on the whole.
The conventional wisdom – that positive limitations are preferable to negative ones – probably remains a safe bet. Practically speaking, in many cases it is possible to rewrite a would-be negative limitation as a positive limitation, for example by reference to specific positive features that of necessity exclude the “unwanted” feature.
1 The case was appealed to the Federal Court of Australia (and subsequently the Full Federal Court), but the validity of the negative limitations was not at issue on appeal.
2 At 
4 See throughout the decision, but e.g. , , , , [22.3] – while also noting that other factors like clarity (EPC Art. 84) may come into the issue: e.g. [22.2], 
5 USPTO’s Manual of Patent Examining Procedure, at 2173.05(i)
Author: Anna Klepacki