An unanswered question of beauty

Instigated by Steven of Flos Carmeli, I've gotten myself in a puzzlement over the nature of beauty. Namely, just what is the relationship, if any, between beauty and prettiness?

We use the same word to indicate the opposite of both, ugly. There are lots of examples of things that are both beautiful and pretty, like the paintings of Monet or Vivaldi's Four Seasons. But, Steven made a list of artworks to be rated, whether they were beautiful or not, and on that list was Picasso's "Guernica."

Now, "Guernica" is not pretty at all. But then, the Guernican Massacre was not at all pretty either, it was a horror. The painting, in all its un-prettiness, does truly re-present that incident to us in all its horror and terror. Is it possible to say something is beautiful when there is no prettiness at all? Even when there is power and truth? I seem to have always believed so, since I've always held "Guernica" as a great and beautiful work --- but, is beauty really the right word?
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