Basic linking

I've been making sites for a while but I never really learned the right way to do things I suppose now is a good time to fix things.

Whenever I linked to a file that was not in the same folder or within a folder directly 'below' it, I would always code the full url. So what is the correct way to link to a file one level up, or two levels up, or even one level up and then down into another folder (you get the point).

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