In typography, overstrike is a method of printing characters that are missing from the printer's character set. It was widely used around early 1990s. The character was created by placing one character on the another one — for example, overstriking "L" with "-" resulted in printing a "Ł" character.
Many font renderers in computer programs invent missing bold characters by overstriking the normal character with itself, slightly horizontally offset. The horizontal offset is essential, since unlike a typewriter, where repeating a letter in exactly the same space will make it darker, most modern printers will not darken repeated "strikes" to the same space. Actual bold fonts are designed with some features thicker and others the same size as a regular font, so the use of this "fake bold" is considered undesirable from a typographic point of view; some high end typesetting programs go so far as to refuse to fake bold in this way.