The Number Sign # or Hashtag

# has uses other than as the first character in social media hashtags

Hashtag symbol. Jeffrey Coolidge/Getty Images

Have you used an octothorpe recently? You have if you've typed a hashtag on a social media site. Octothorpe is one name for the number symbol, also called a pound sign, number sign, hash, hashtag, comment sign, hex, cross, square, punch mark, grid, and others.

On a standard US keyboard, the # sign is located on the 3 key, where it's accessed while holding down the Shift key in Windows. It consists of two slightly slanted parallel lines crossed by two horizontally parallel lines.

You can think of it also as an italicized tic-tac-toe game.

Uses of the # Sign

Despite the relatively recent explosion in popularity of the hashtag on social media, the number sign is most often used in front of a numeral in place of the word number, such as "#1" instead of "number 1"—for example, Students need to bring a #2 pencil to class to complete quiz questions #1 to #10. 

Other applications include the following:

  • As a proofreading mark designating the insertion of space between lines of text or space between words. Proofreaders insert a # in the text where additional space is needed.
  • As a telephone key, usually referred to as the pound key.
  • In computing, the # (here called a hash) designates comments or commands in a programming language, a permalink in a blog, and other uses.
  • A series of three # signifies the end of a press release: ###.
  • In music, the symbol is graphically similar to a sharp note.
  • As the previously mentioned designator in social networking known as the hashtag. Hashtags are attached to keywords or phrases to identify messages on a particular topic—for example, #thoughtco.

Origins of the Number Sign

Though its true origin has yet to be verified, one legend holds that the pound sign comes from the symbol for the Roman term libra pondo, which means "pound weight." You can see the resemblance.

Though the symbol used to be more complex, it was simplified in favor of two horizontal strokes crossing with two forward slashes. One 1896 typewriter manual indeed referred to it as the "number mark."