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Where there is a relevant prior art document which discloses most, but not all, of the features of a claim, examiners must determine whether the differences are in respect of essential features of the claim.

The issue of determining essential features was discussed in Catnic Components v Hill and Smith Ltd (1982) RPC 183 at page 228:

"... I think that one can venture upon the following generalisations on the question of essentiality. (1) If that feature of the claim which is under consideration is in fact essential to the working of the claimed invention, then it must be an essential feature of the claim. (2) If the feature is not in fact essential to the working of the claimed invention, the applicant for a patent may nevertheless have made it an essential feature of the claim, that is to say, he may by the terms of the claim as properly construed have clearly limited his claim to a subject matter having that particular feature. If so, that feature will be an essential feature of the claim and anyone who makes a product or carries out a process which has all the features of the claim except that particular feature will not infringe the claim. But (3), all claims are not perfectly framed. Sometimes a draftsman may include some feature in a claim, either explicitly or by implication, which is not in fact essential to the working of the claimed invention and which the applicant has not by the terms of his specification and claim clearly indicated as a feature which he regards as an essential feature of his monopoly. In such a case an alleged infringer may be held to have infringed the claim notwithstanding that his product or process does not incorporate the feature in question or substitutes some equivalent for it. (4) The fact that a claim incorporates a particular feature does not alone suffice to make that feature an essential one. If this were not so, no feature of a claim could ever be inessential, but the speeches in Rodi and Wienenberger* all assume that a claim may include an inessential feature."

* Rodi and Wienenberger v Henry Showell Ltd (1969) RPC 367.

In order to determine the essential features of a claim, examiners must construe the specification using the general rules outlined in Rules of Construction.  In particular, it is necessary to:

  • construe the specification as a whole;
  • construe the specification as a skilled addressee;
  • read in the light of the common general knowledge;
  • construe as at the priority date of the relevant claim;
  • purposively construe the specification; and
  • determine the true ambit of the claim, including proper consideration of words of purpose.
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