There are two main aspects of the public domain:

* What forms the public domain?
* How can we profit from the public domain?

At this point you must have a dozen questions: How does the copyright law work? How do you find out if a work is copyrighted or not? Where do patents figure into all this? How do we distinguish between ‘ideas’ and the work itself? What about derivative works?

Before we tackle the meaty issues such as copyright law and the other questions, let’s see how something can fall into the public domain.

> All generic information such as facts, figures, scientific knowledge (but not inventions), mathematical formulas, ideas, etc. The work cannot be copyrighted. This includes things such as ideas, facts, theories, mathematical or scientific formulas, and also simple things like a list of ingredients or components. Therefore you cannot copyright the Theory of Relativity, or the knowledge of how to make a Molotov Cocktail (a type of a home-made bomb).

> All publications by the government and its agents. Specifically, the US copyright law prevents such publications from being ‘copyrighted’ (this has a lot to do with the definition of copyright and copyrightable work) and thus they are directly part of the public domain.

> Previously copyrighted works that have passed into the public domain due to expired copyright or other reasons. It is quite possible to find works published in the late 1800s and the early 1900s (late 19th century and early 20th century) whose copyright has expired and has not been renewed under current copyright laws.

> Work that is placed into the public domain by its owners. The copyright owner dedicated the work into the public domain. This is more common than you think. An often-quoted example is that of freeware software or freely distributed source code. Sometimes artists tend to dedicate their work to the public domain as well.

> Work created and published before there were copyright laws. This distinction is very important and is the sole reason why the works of Shakespeare are considered public domain while Darwin’s ground-breaking book, Origin of Species, is protected by copyright.

> The work was never copyrighted – This is a tricky issue. Logically, if a person publishing his work does not acquire copyright, then by reason it is automatically part of the public domain. However, all is not as simple as it first seems.