To be continued - what's a continuation patent?

Dear Rich: There is an invention for a sanitation tube system that will expire this year. However, I found another invention that had been issued three years after the first one which was related to an adaptor to fix the original tube from the same inventor mentioning that this invention is continuation of the earlier patent. Is it legal for me to make a tube based on the first invention? Does the word "continuation" in the second patent mean that I can't touch the first patent unless the second one is expired? Can the invention be issued twice? First, we'll answer your last question. No, an invention cannot be issued twice -- that's known as double patenting and is illegal. And of course, once a patent expires you can make, use, or sell the invention covered by its claims and not worry about getting chased. 

What's a continuation? The Dear Rich Staff went to the always-helpful (and now-legendary) David Pressman, author of Patent It Yourself (insert FTC disclaimer), for some help with your other questions. David explained that a continuation is supposed to replace the parent application. A parent and its continuation are supposed to cover the same invention and generally both do not issue (patent lingo for the official grant of the patent). 

The problem is ... However sometimes the USPTO issues the parent and a continuation. They usually have somewhat different claims and the continuation usually expires the same day as its parent application, so you can make, use, sell the invention covered by its claims with impunity once the continuation expires. A "terminal disclaimer" is supposed to be filed in the continuation so it will not last beyond the expiration of the parent. If this situation occurs, you'll need to consider infringement of the continuation separately from its parent case. That is, one does not affect the other. 

Is this confusing enough for you? We're getting a little light headed here. Summing up (if that's the right phrase), the continuation may have a terminal disclaimer and may or may not cover the same invention as the parent. Both may be invalid for double patenting if a terminal disclaimer was not filed. David advises that since this is such a complicated matter, you might want to have a patent attorney review the patents. Why didn't we just say that in the beginning? We'll ponder that and get back to you.