The major keys in order are:
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I can’t see the picture of the painting and its artist. But the purpose of equestrian portraits in history is that they demonstrate elements kept on horsebacks.
Artists emphasized classical subjects in the human form, and just new techniques for showing subjects more realistically
this is more of a personal question, i guess one example would be to study on the subject youre teaching, but like I said, this is more of a decision you would make