As an inventor, one of the things I did not appreciate is how to maximize the value of a patent family. I suspect that one reason for this is that the attorney with whom I did much of my work focused on drafting the patent and nursing it through the prosecution process (note: “prosecution” in this use means “getting it through the patent process” not “enforce it.”)
Since that time, I have worked with litigators and patent brokers. Litigators taught me that patent owners could use one trick to “keep the patent prosecution alive,” which means that the patent owner continues to submit new claims against the original specification. From a litigation perspective, the patent owner can file new claims using the original specification and, if successful, have a patent that can then be enforced against potential infringers. Brokers taught me that the value of a patent is much higher if a potential buyer can file new claims on the original specification because it makes the patent family far more valuable in potential litigation.
Multiple patents against the same specification share a common priority date and a common expiration date. Usually, multiple patents against the same specification are considered a “family” of patents.
One good example of this is a well-known patent owned by Leland Stanford Jr. University (most people call it “Stanford,” though.) This is US Patent 6,285,999. It is a seminal patent because it provides the original description (“teaching” in patent parlance) of ranking web pages based upon how many other web pages are referenced. The algorithm is commonly used in my area of computer science (“systems”) and is referred to as PageRank. In addition, PageRank is well known enough that it has its own Wikipedia page.
On January 10, 1997, the original specification was filed as provisional application US3520597P. Thus, this is the “priority date” of the subsequent patent applications because they are all based upon the same common specification.
If you review the history of this patent, the first actual application was filed on January 9, 1998, the last day the provisional application was valid (that period of validity was one year; as far as I know, it still is.) The patent (6,285,999) was granted on September 4, 2001. The “Notice of Allowance” from the patent office was issued on April 23, 2001. The patent issue fee was paid on July 11, 2001. The second application was filed on July 2, 2001.
Because the second application was filed before the patent was issued, it “continued” the application process against the original specification. This process was repeated ten additional times. Thus, 12 different applications were filed against the same specification. The most recent application was awarded a patent on May 13, 2014 (8,725,726).
If there is no active continuation application on file with the USPTO when a patent issues, that specification is complete. Therefore, it is now part of the “prior art,” and no future patent claims can be inferred from that original specification.
Bottom line? Suppose you want to maximize the profit potential of your patents (as an inventor). In that case, it is good to keep an application open as it allows you (or a subsequent owner of the patent) to file an additional application focused on specific claims that can then be used to protect your invention.
I realize some people may not be familiar with Pagerank. However, this algorithm is the basis of the technology that launched Google. Larry Page, the inventor, was a graduate student at Stanford at the time. Thus, this is likely one of the most valuable patents ever granted.