The works during the 19th century were very interesting as they often made art symbolic.
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This is an example of an old piece of art, it is a girl singing to herself whilst she drowns/before she drowns. She has been illustrated this way to show that she didn't think there was a point in struggling.
This is typically the rule which a lot of artists use when they want to have a quite neat composition. This is a great way to balance the focus within the image. However, if you were to do a portrait you could also use this to figure out proportions of the face.