How the paragraph came to be

According to Jan Tschichold, in an essay titled "Why the beginnings of Paragraphs Must be Indented" ...The medieval paragraph symbol ¶ originally could also appear in the middle of running lines and was colored. It signified the beginning of a new group of sentences. During the late Middle Ages such groups of sentences were introduced with a new line, but the habit remained of beginning the new group with the symbol for paragraphs, usually written in red. Some of the early printers even cut it as a type sort and printed it in black. Previously, though, it was inserted by hand in red by the rubricator (whose job description stems from the color: rubrum = red). The space for the symbol had to be left blank by the typesetter. But rubrication often did not take place, and it was found that the em quad indention or indent, as we call this empty space today, was sufficient by itself to define a new group of sentences, even without the red symbol. See his books for advice on using paragraphs, and lots more.