In the olden days before the AIA (America Invents Act) US Provisional applications provided inventors proof of invention. Back then proof of invention would establish the presumptive priority date for the whole invention. The details could be fleshed out in the subsequent filed non-provisional application.

After the AIA, the so-called first-to-file rule took effect where priority date is set based on the date you file the first application (provisional or non-provisional). But for a few exceptions, the first to file wins, first to invent does not apply anymore.

Now, US Provisional patent applications only protect the subject matter that is in the fours corners of the document. The only time we file provisional applications now, is if it there is a bar date emergency or the inventors have not had time to approve the draft non-provisional application before the bar date.

@octoberfranklin. Agreed. You are correct. The law is more nuanced. Mine was intended to be a short example for a layperson. I thought a nuanced 10,000 word response might be overkill in this forum. :-) My intention was to point out that a short provisional patent application prepared by a non-expert in patent law may end up not providing the intended protection.