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The specification may indicate that a special meaning is to apply. A special meaning may only prevail over the plain and unambiguous meaning if the special meaning is clearly expressed. This is known as the dictionary principle.

The nature of the dictionary principle is reflected in British Thomson-Houston Company Ld. v Corona Lamp Works Ld. (1922) 39 RPC 49 at page 67, where Viscount Haldane stated:

"We have to scan the Specification with the closeness which is required in the case of any instrument conferring a monopoly, but, subject to this, all we can legitimately do is apply the ordinary rules for the construction of written instruments. One of these, which is relevant in the case before us, is that the instrument must be read as a whole. The Claiming Clauses, for example, are not to be taken as standing in complete isolation. For if the Patentee has used in these clauses expressions which he has already adequately interpreted in the body of his Specification, he is entitled to refer to the Specification as a dictionary in which the meaning of the words he uses has been defined."

It must be clear that the specification is setting up a dictionary. Thus, in Minerals Separation North America Corporation v Noranda Mines Ltd. (1952) 69 RPC 81 at page 93, Lord Reid stated:

"The Appellants contend that there is in the earlier part of the specification a definition of the word "xanthate" as used by the Patentee which is in effect a "dictionary" and that, as the Patentee has shown that he intends the word to be understood in a limited sense throughout, that limited sense ought to be attached to the word when it occurs in claim 9. Their Lordships do not doubt that it is possible for a patentee to make his own dictionary in this way. If he has put something in the earlier part of the specification which plainly tells the reader that for the purpose of the specification he is using a particular word with a meaning which he sets out, then the reader knows that when he comes to the claims he must read that word as having that meaning. But this is an awkward method of drafting ... and it is in all cases incumbent on a patentee who chooses to adopt this method to make his intention plain to those who read the specification."

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