Through all these changes, nevertheless, there remained a broad consensus theory of what `free software' or `open source' is. The clearest expression of this common theory can be found in the various open-source licenses, all of which have crucial common elements.

In 1997 these common elements were distilled into the Debian Free Software Guidelines, which became the Open Source Definition. Under the guidelines defined by the OSD, an open-source license must protect an unconditional right of any party to modify (and redistribute modified versions of) open-source software.

Thus, the implicit theory of the OSD (and OSD-conformant licenses such as the GPL, the BSD license, and Perl's Artistic License) is that anyone can hack anything. Nothing prevents half a dozen different people from taking any given open-source product (such as, say the Free Software Foundations's gcc C compiler), duplicating the sources, running off with them in different evolutionary directions, but all claiming to be the product.

This kind of divergence is called a fork. The most important characteristic of a fork is that it spawns competing projects that cannot later exchange code, splitting the potential developer community. (There are phenomena that look superficially like forking but are not, such as the proliferation of different Linux distributions. In these pseudo-forking cases there may be separate projects, but they use mostly common code and can benefit from each other's development efforts completely enough that they are neither technically nor sociologically a waste, and are not perceived as forks.)

The open-source licenses do nothing to restrain forking, let alone pseudo-forking; in fact, one could argue that they implicitly encourage both. In practice, however, pseudo-forking is common but forking almost never happens. Splits in major projects have been rare, and are always accompanied by re-labeling and a large volume of public self-justification. It is clear, in such cases as the GNU Emacs/XEmacs split, or the gcc/egcs split, or the various fissionings of the BSD splinter groups, that the splitters felt they were going against a fairly powerful community norm [BSD].

In fact (and in contradiction to the anyone-can-hack-anything consensus theory) the open-source culture has an elaborate but largely unadmitted set of ownership customs. These customs regulate who can modify software, the circumstances under which it can be modified, and (especially) who has the right to redistribute modified versions back to the community.

The taboos of a culture throw its norms into sharp relief. Therefore, it will be useful later on if we summarize some important ones here:

In the remainder of this essay, we shall examine these taboos and ownership customs in detail. We shall inquire not only into how they function but what they reveal about the underlying social dynamics and incentive structures of the open-source community.