Birth Certificate
Death Certificate
Marriage Certificate
Divorce Decree
Social Security Application
Draft Registration
Plus there could be more that are a bit more unusual. For instance, I have a copy of the papers for one relative that officially changed his name.
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So I like to keep the originals (If I have originals) in plastic sleeves in a binder.
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I'll have additional paper copies of them stored with the originals, and also in other family-specific binders and folders. Also, for you eagle-eyes out there, yes, I do keep a copy of Family Tree Maker 16 on my computer because I love the deliciously decadent feeling it gives me when it tells me my grandmother is also my cousin. The newer versions just don't do that (unless a really new version does and I just don't know about it, tell me if it does!!!). So it stays.
Back to the topic at hand...of course, there is the magical Redundancy Rule that saves me every time - it means everything has been scanned and stored on my computer's hard drive, on an external hard drive, and also burned onto a CD - one copy is stored at my house, and a 2nd copy is forced upon my father to store at his house in case my house ever detaches itself from its foundations and collapses in on itself like the house in Poltergeist. I hope I didn't just ruin the movie for you. Also, because I like to take things to an extreme (in case you didn't already notice), I store my electronic copies as both TIFF (for long term storage) and JPG (for actual use - emailing, posting).
Finally, I keep an inventory of what I have. Why? Because yes, I have actually ordered the same Social Security Application twice in the past. And you who have done that know it is not an insignificant price or waiting period. So when I realized I did that, I decided to keep a spreadsheet inventory.
I'm not saying my way is the right way - it's what works for me and I'm always open to new ideas!!!