Andrew Rens reminds us of an important distinction. He starts by quoting James Boyle:

“The term “commons” is generally used to denote a resource over which some group has access and use rights—albeit perhaps under certain conditions. … Some would say it is a commons only if the whole society has access. That is the view I will take here. The other difference between public domain and commons is the extent of restrictions on use. Material in the public domain is free of property rights. You may do with it what you wish. A commons can be restrictive. For example, some open source software makes your freedom to modify the software contingent on the condition that your contributions, too, will be freely open to others…So these are working definitions of public domain and commons. But why should we care? Because the public domain is the basis for our art, our science, and our self-understanding. It is the raw material from which we make new inventions and create new cultural works.” (p39)

This distinction is not only clear to lawyers, David Bollier, a journalist turned technology policy expert has also released a book under an open licence entitled Viral Spiral (free download) and he describes the difference in non lawyer terms here:

“The public domain is an open-access regime available to all; it has no property rights or governance rules. The commons, however, is a legal regime for ensuring that the fruits of collective efforts remain under the control of that collective. The GPL, the CC licenses, databases of traditional knowledge, and sui generis national statutes for protecting biological diversity all represent innovative legal strategies for protecting the commons.”

Being a lawyer I’d want to complicate Bollier’s description just a little, the public domain is subject to governance rules, rules which allow the incorporation of the public domain into an all rights reserved intellectual property claim, but don’t permit the exclusion of others from that element of the public domain. So for example one can use a mathematical formula in a patent, the patent can exclude others from making a similar invention but they can use the formula elsewhere. Another example would be that one can copy a text in the public domain for example Bleak House, and claim copyright (a peculiar type of copyright called a published edition) not in the copy but in a reformatted version. As a result someone can’t run off hundreds of copies of your new edition although of course someone else can put out their own printed version of Bleak House.”