Hacking through the Software Patent Thickets
by Glyn Moody
Most people in the hacking community are well aware that patents represent one of the most serious threats to free software. But the situation is actually even worse than it seems, thanks to the proliferation of what are called patent thickets. To understand why these are so bad, and why they represent a particular problem for software, it is necessary to go back to the beginning of patent law.
As their name implies, patents are about making something “patent”, or public, so that others can use and build on an invention – but only after a certain period, during which time the patent-holder has a monopoly right (what kind of right is discussed below). Indeed, the origin of British patent law – and of many anglophone countries that base their law on this tradition – is the 1623 Statute of Monopolies.
The idea of patents was to ensure that new discoveries were not kept secret. Allowing a time-limited government-granted monopoly was the quid pro quo for revealing the details of the discovery to everyone. Even though monopolies have always been regarded as a Bad Thing for society, at a time when inventions were relatively rare it made sense to offer this powerful incentive. Without it, the danger was that new knowledge would remain locked away as trade secrets, which would have been detrimental to the overall advancement of technology.
It's important to note that the monopoly granted by governments is not a right to use that invention; as Wikipedia explains:
A patent is not a right to practice or use the invention. Rather, a patent provides the right to exclude others from making, using, selling, offering for sale, or importing the patented invention for the term of the patent, which is usually 20 years from the filing date  subject to the payment of maintenance fees.
That is, at the heart of the patent is the power to exclude others from using it. This has a very important consequence:
A patent being an exclusionary right does not, however, necessarily give the owner of the patent the right to exploit the patent. For example, many inventions are improvements of prior inventions that may still be covered by someone else's patent. If an inventor takes an existing, patented mouse trap design, adds a new feature to make an improved mouse trap, and obtains a patent on the improvement, he or she can only legally build his or her improved mouse trap with permission from the patent holder of the original mouse trap, assuming the original patent is still in force. On the other hand, the owner of the improved mouse trap can exclude the original patent owner from using the improvement.
In the early days of patents, this wasn't much of a problem. Most inventions were self-standing, and therefore didn't interact with earlier patents (not least because there were so few of them). But as time went on, and more patents were issued, so the tendency to interact increased, often requiring permission from holders of pre-existing patents in order to implement them.
Another important factor was the increase in complexity of inventions over the years. Whereas the first patents were for an entire machine or complete process, later inventions necessarily tended to be for parts of a machine or process (because the key ideas and techniques were either already patented, or by now in the public domain).