Sheppard J summarised the rules of construction for a patent specification in Décor Corp v Dart Industries 13 IPR 385 at 400. This summary has been referred to in numerous subsequent judgements and was endorsed by the Full Federal Court in Pfizer Overseas Pharmaceuticals v Eli Lilly  FCAFC 224. The rules are as follows:
The claims define the invention which is the subject of the patent. These must be construed according to their terms upon ordinary principles. Any purely verbal or grammatical question that can be answered according to ordinary rules for the construction of written documents is to be resolved accordingly.
It is not legitimate to confine the scope of the claims by reference to limitations which may be found in the body of the specification but are not expressly or by proper inference reproduced in the claims themselves. To put it another way, it is not legitimate to narrow or expand the boundaries of monopoly as fixed by the words of a claim by adding to those words glosses drawn from other parts of the specification.
Nevertheless, in approaching the task of construction, one must read the specification as a whole.
In some cases the meaning of the words used in the claims may be qualified or defined by what is said in the body of the specification.
If a claim be clear, it is not to be made obscure because obscurities can be found in particular sentences in other parts of the document. But if an expression is not clear or is ambiguous, it is permissible to resort to the body of the specification to define or clarify the meaning of words used in the claim.
A patent specification should be given a purposive construction rather than a purely literal one.
In construing the specification, the Court is not construing a written instrument operating inter partes, but a public instrument which must define a monopoly in such a way that it is not reasonably capable of being misunderstood.
The body, apart from the preamble, is there to instruct those skilled in the art concerned in the carrying out of the invention; provided it is comprehensible to, and does not mislead, a skilled reader, the language used is seldom of importance.
Nevertheless, the claims, since they define the monopoly, will be scrutinized with as much care as is used in construing other documents defining a legal right.
If it is impossible to ascertain what the invention is from a fair reading of the specification as a whole, it will be invalid. But the specification must be construed in the light of the common knowledge in the art before the priority date.
See also Flexible v Beltreco 49 IPR 331 from paragraph 70, where Hely J expressed the rules slightly differently.
In addition to the above rules, the Full Federal Court in Pfizer Overseas Pharmaceuticals v Eli Lilly  FCAFC 224 referred to a further three (from Nesbit Evans Group Australia Pty Ltd v Impro Ltd (1997) 39 IPR 56):