What is the nonobviousness requirement?

One of the requirements for obtaining a patent is that an invention cannot have been obvious to someone with skill in the invention’s field. In other words, this requirement is intended to ensure that the invention is an actual advance or step forward, rather than a tweak or simple change to inventions already in existence. Determining whether an invention is obvious is one of the most difficult aspects of patent law. The PTO patent examiner typically seeks to make this determination by looking at past patents related to the patent application to see if any of those past patents, combined, contain all of the features of the invention in the patent application. If the examiner finds such past patents, he is likely to reject the patent application as being obvious. 

The distinction between this nonobviousness requirement and the novelty requirement is that here the examiner compares the patent to a combination of prior art while, when looking at the issue of novelty, the examiner compares the patent application separately to each piece of prior art.

When an examiner rejects a patent application based on this nonobvious requirement, the inventor will often argue that, although there are two piece of prior art which are related to the invention, it would not have been obvious to combine them in the way his invention does. This argument is sometimes persuasive, but not always.