The concept of “Reduction to Practice” doesn’t actually matter these days, unless you have a US patent application that was filed before March 16, 2013. US patent applications filed before that date are granted to the “first-to-invent,” whereas applications filed after that date are granted to the “first-inventor-to-file” a patent application. The law that made the change is known as the “America Invents Act” or “AIA.”
In the pre-AIA days, a person who was the second to file a patent application for a particular invention could prove that he/she was actually the first person to invent it by proving that they had “actually reduced the invention to practice” on some date before the person who was first to file. Under the pre-AIA rules, only a patent application with an enabling disclosure qualifies as a “constructive reduction to practice.” An “actual reduction to practice” requires physically making a working model of the invention. (The full rules of establishing first-to-invent are much more complicated, but are unimportant here.)
But, as I said above, it doesn’t actually matter unless you already have a pre-3/16/13 patent application pending.
There is a recurring theme in which inventors hope that a witnessed invention disclosure document will somehow be proof that the he/she invented something as-of a particular date. This is commonly known as the “Poor Man’s Patent.” Variations include mailing the disclosure to yourself in a sealed envelope, putting the disclosure in a social media or blog post, or giving the disclosure to a lawyer or relative for safe keeping.
In the post-AIA US patent system, Poor Man’s Patents are worthless.
Under the current rules in the US (and in the rest of the world), the only way to establish a priority date that will be meaningful for the purposes of getting a patent is to file a patent application.
The risk of not doing so is mainly that someone else might file a patent application before you and obtain a patent for your invention. The AIA also created a so-called prior-user defense to patent infringement, which you might also want to discuss with a patent attorney.
If you have an invention that you want to own, then you should talk to a patent professional about how best to achieve your goals. Feel free to contact me.
this post is based on my answer to the Quora question: