* Does one use multiple sources for same event?

[AdrianBruce]

...
Tertiary source = GRO certificate copied by hand or typed as a transcript of the quarterly return
...

So far as I can see, "tertiary source" (yes, source) is a term used in academic research https://en.wikipedia.org/wiki/Tertiary_source, and "the great and the good" in genealogy tend not to use it. (Logical though it might be). I think it's one of several potential differences btw academic history and genealogy in sourcing - we don't even agree on the definition of primary!

[arthurk]

In fact a GRO certificate may even be a tertiary source:

I'd call a GRO certificate a derivative source (a copy of an original, or as you point out, possibly a copy of a copy of the original)....

As you surmised, I was using tertiary in the sense of a copy of a copy of the original, but maybe, as Adrian says, that isn't the right or best term to use. Apologies if I've confused anyone.

[ColeValleyGirl]

As you surmised, I was using tertiary in the sense of a copy of a copy of the original, but maybe, as Adrian says, that isn't the right or best term to use. Apologies if I've confused anyone.

We all have different terminology, derived from different experiences/education. I doubt you or I have confused anyone, we've just illustrated the differences that exist.