"Oh, what a mess!"
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The Art (D'ni: regestO, regestoy) was a term used by the D'ni to refer to mastership of the Skill required in order to write Descriptive and Linking Books to other Ages. The D'ni knew the Art and passed the knowledge on to their children, however it is not exactly known how they - or the Ronay, from which they descended - learned it in the first place.
One did not have to be D'ni to learn the Art; other species of sufficient intelligence can learn how to write just as well. By the same token, having D'ni ancestry did not make someone a good writer if they had no artistic talent.