Roubo was many things, and apparently one of them was a polished wordsmith (or he had a great ghostwriter), although in this instance he cited another author as the source for this expression..
Moldings are part of the ornaments of architecture (and consequently of woodworking, which makes part of the latter), or better said, they are distinct features, which serve to give to different works a character of richness or of simplicity relative to the different subjects that you are drawing. One can therefore compare moldings to letters that are used in writing, which by the combination of different characters, form an infinity of words according to the diversity of languages.
Now that is both brief and elegant!
It will be included in the second chapter in our upcoming To Make As Perfectly As Possible: Roubo on Furniture Making.