In many Web browsers and other computer programs, when converting text between encodings, it may not be possible to map some characters into the target character set. In this situation it is common to replace each unmappable character with a question mark <samp>?</samp>, inverted question mark <samp>¿</samp>, or the Unicode replacement character, usually rendered as a white question mark in a black diamond: . This commonly occurs for apostrophes and quotation marks when they are written with software that uses its own proprietary non-standard code for these characters, such as Microsoft Office's "smart quotes".