A letter that is accidentally placed upside down is called a fly's head in printer language . Instead of the desired character, the base of the letter creates two rectangles in the print image, which are separated by a gap created by the casting groove. The printed image thus resembles the compound eyes of insects.
Letters intentionally placed upside down were sometimes used as placeholders in the sentence to create space for the letter that was actually to be used at this point, which then had to be inserted later. Sometimes this procedure was also chosen to mark text passages for later corrections. Letters that are intentionally used incorrectly are not referred to as a fly's head in printer language, but as a blockage .
However, if space was not only required for individual characters, but also a larger area for material to be added later such as clichés or for separately set sentences such as tables or advertisements, the sentence had to be temporarily filled with dummy material . This process was called freeing .