There’s a specific term for this, and even an entire Wikipedia article: In typography, rivers (or rivers of white) are gaps in typesetting which appear to run through a paragraph of text due to a coincidental alignment of spaces. They have this example:


Dan Bron's comment is exactly the features you're describing. You can consider this a typography example of negative space: Negative space, in art, is the space around and between the subject(s) of an image. Negative space may be most evident when the space around a subject, not the subject itself, forms an interesting or artistically relevant shape, and ...

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