Patents have a term of 20 years from the filing date. After this term, the technology enters the public domain for all to use. This balances incentivizing innovation during the patent term with allowing further innovation and broader access after the term expires.
Often, clients ask how they can extend or otherwise expand upon the patent coverage of their core products or technology. The answer here is easier said than done: one must develop a new and non-obvious innovation compared to the existing technology.
The ubiquitous Pez Dispenser provides a good example of this. Pez Dispensers have been on the market in the US since the 1950s. Despite this, there is currently an in-force patent on Pez Dispensers: US 7,523,841. This patent focuses on an improved magazine for the candy tablet dispenser. This magazine was determined to be new and non-obvious over the existing Pez Dispensers. While the earlier technology from Pez is indeed in the public domain, the current patent allows Pez to maintain patent protection on their improved dispenser, and market the device as a patented product despite the original technology dating back to the 1950s or earlier.
Therefore, through continued innovation, inventors are able to extend their patent coverage to their new features of core technology.