Generally you don. At least not exactly. I got a patent for font ID based on the fact that the collective s of glyphs in a font are unique There are some exceptions. Monospaced or writer style fonts have all their glyphs the same . Typically it is half or 6% of the point size (or em in font speak). So their s are easily matched. It is possible to use axis-based font technology (such as multiple master or OpenType variations) to faux another font. This is an approach that Adobe Acrobat used to do when document file sizes were more of an issue you could only the character s in the PDF and Acrobat would simulate the look of the document without using the actual font. Almost nobody wants that any more so it is more an obscure theoretical thing than amon use case. ordered-list But if you have an arbitrary randomly-selected font that isn monospaced and you want an already-existing italic font that matches its character s generally your odds are not good.