The question mark ? is a punctuation mark that indicates an interrogative clause or phrase in many languages. This earliest question mark was a decoration of one of these dots, with the 'lightning flash' perhaps meant to denote intonation, and perhaps associated with early musical notation like neumes. Another possibility is that it was originally a tilde or titlo, as in ·~, one of many wavy or more or less slanted marks used in medieval texts for denoting things such as abbreviations, which would later become various diacritics or ligatures. Over the next three centuries this pitch-defining element (if it ever existed) seems to have been forgotten, so that the Alcuinesque stroke-over-dot sign (with the stroke sometimes slightly curved) is often seen indifferently at the end of clauses, whether they embody a question or not.

In the early 13th century, when the growth of communities of scholars (universities) in Paris and other major cities led to an expansion and streamlining of the book-production trade,[6] punctuation was rationalized by assigning Alcuin's stroke-over-dot specifically to interrogatives; by this time the stroke was more sharply curved and can easily be recognized as the modern question mark. According to a 2011 discovery by Chip Coakley, a Cambridge University manuscript expert, Syriac was the first language to use a punctuation mark to indicate an interrogative sentence. The Syriac question mark, known as the zagwa elaya ('upper pair') has the form of a vertical double dot over a word.

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 ?, inverted question mark ¿, or the Unicode replacement character, usually rendered as a white question mark in a black diamond: U+FFFD � REPLACEMENT CHARACTER. 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'.