Patent Myths - Changing the Invention
The marginally better remark I hear is: "I've heard that there's no point in getting a patent because someone just has to change it a little bit and they can get around the patent". This remark is marginally better because there are many patents out there which define a patent so narrowly that a small variation will be sufficient to avoid infringement.
But sarcasm and condescension aside, what is the proper response to such a comment?
The protection that is afforded by a patent is governed by a set of claims that appear at the end of the written part of the patent specification. Apart from the US, that's just before the drawings. Each claim is usually a sentence in the form of a definition. Have a browse around google patents and you will find plenty of examples of claims. There's actually an 1885 patent there for a pair of scissors...
Claims are usually divided into two types. These are independent claims and dependent claims. There is such a thing as an omnibus claim, but I won't get into that now.
Independent claims, as the name suggests, exist independently and define the invention in its broadest terms. So let's say an invention, in its broadest sense, has three features or integers (to use patent jargon). Call these integers A, B and C. Then an independent claim could read: "A widget comprising Integer A; Integer B attached to Integer A; and Integer C which is rotational with respect to Integer B". We'll call that Claim 1. Let's say the inventor has conceived an optional Integer D. Then Claim 2 could read: "A widget as claimed in claim 1, in which Integer D is hinged on Integer A". Of course, this is a ridiculously simple example because it does not consider many other types of claims, such as process or method claims. But I think it will do for the purposes of explaining my point.
During patent prosecution (see here for more on patents and the basis for their existence) your patent attorney will effectively present Claims 1, 2 etc. to the patent examiner requesting acceptance and subsequent grant of a patent - see the link above. The allocated examiner carries out a search and possibly discovers that the widget with integers A, B and C was published before the date on which you filed your patent application. So, you amend your claims to incorporate integer D into a fresh claim 1.
In some fields, there is a large amount of patent activity going on. I recall when I started in this profession the firm that I was with kept a file with a large variety of toothbrushes having built-in toothpaste tubes. Another file had cigarette packets with match packs attached (yes, it was that long ago...). In such fields, it is likely that the patent examiner will find plenty of "prior art" documentation. It follows that the broadest claim you end up with might be quite narrow. There can be situations where the protection is so narrow, it's not worth prosecuting the patent application any further.
In addition, there are text books and hundreds of cases dealing with the manner in which claims are interpreted in order to assess the scope of protection that they afford the patentee.
So, I hope that you can see that there is no "rule of thumb" that you can use to gauge how much a copy can be altered to avoid infringement.