This project is a typographical research about the .notdef glyph, also known as the Not Defined glyph, or U+FFFD in Unicode. It serves as a placeholder for missing characters, making it clear when a character is not present in a font file. U+FFFD takes this into account and questions the gap in typefaces. From up to 500 typefaces, 87 unique approaches are shown in the book. Ultimately, to what extent does U+FFFD symbolise linguistic gaps? Who creates them and who uses them as an opportunity to be filled?