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During the 18th century, British caricaturists changed the shape of speechballoons from gothic speech-bands or flags into fluffy balloons, our modern speechballoons. I'm using the word speechballoon as the general, inclusive term. (The gothic form of speechballoons are speechbands, flags, scrolls or sheets of paper, the modern form of speechballoons are balloons, but also little rectangles, often rounded at the edges, or simply little blocks of text above the heads of the speaker etc, etc).