You’re looking at it right now on your keyboard. It’s that little grid of four intersecting lines sitting above the 3. Depending on how old you are, where you live, or what you do for a living, you probably have a completely different name for pound symbol than the person sitting next to you. It's a shapeshifter.
To a Gen Zer, it’s a hashtag. To a software engineer, it’s a hash. To your grandmother, it’s the pound sign she uses to check her bank balance over the phone. But if you talk to a cartographer or a very specific type of copy editor, they might call it an octothorpe. Seriously. That’s a real word.
The history of this symbol is a mess of medieval Latin shortcuts, telegraph lingo, and 1960s corporate marketing. It’s one of the few characters in the ASCII set that carries a massive weight of cultural baggage. It wasn't always a social media tool. For centuries, it was just a messy way of saying "weight."
The Latin Roots of the Libra Pondo
The journey started with the libra pondo. In ancient Rome, this meant "a pound by weight." When medieval scribes were rushing through manuscripts, they didn't want to write out the whole phrase every time they recorded a shipment of grain or gold. They shortened it to lb.
But scribes were fancy. They started adding a horizontal bar across the top of the letters—a "tittle"—to show it was an abbreviation. If you write a cursive lb with a line through the top fast enough, over and over, the loops disappear. The vertical lines of the l and b straighten out. The horizontal bar doubles. Suddenly, you have a 2,000-year-old name for pound symbol that looks exactly like the # we see today.
Sir Isaac Newton actually used a version of this in his handwritten notes. If it was good enough for the guy who discovered gravity, it was good enough for the rest of the world. By the time the printing press took over, the "lb" with a strike-through had morphed into a distinct character used almost exclusively in commercial ledgers.
Why Do We Call It a Hash?
If you’re in the UK, Ireland, or Australia, calling it a "pound sign" will get you funny looks. To them, the pound sign is £. It’s money. Calling a # a pound sign is like calling a comma a dollar sign; it just feels wrong.
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The term "hash" likely comes from "hatch," as in "cross-hatching." It’s a descriptive name. It looks like a series of hatches or a small fence. By the mid-20th century, British military and telecommunications workers were using "hash" to describe the symbol on specialized equipment. It’s punchy. It’s short. It’s efficient.
When computer programming started gaining traction in the 1960s and 70s, "hash" became the standard technical name for pound symbol. If you were writing C code, you were using a "hash-define" directive. You weren't "pound-defining" anything. This linguistic divide between the US and the rest of the English-speaking world created a weird tech gap that still exists today in coding documentation.
The Mystery of the Octothorpe
This is where things get weird. In the early 1960s, Bell Labs was busy reinventing the telephone. They were moving away from rotary dials—which were slow and mechanically temperamental—to Touch-Tone dialing. The engineers realized they needed two extra buttons to round out the keypad to a 3x4 grid. They chose the asterisk (*) and the #.
But they didn't have a formal name for the #.
According to phone company lore, an engineer named Don MacPherson decided the symbol needed a "pro-standard" name. He reportedly combined "octo" (for the eight points at the ends of the lines) with the name of Olympic athlete Jim Thorpe. Others claim it was "octopede," but "octothorpe" is the one that stuck in the Bell System manuals.
It was a total fabrication. A corporate prank that became a legitimate dictionary entry. For decades, if you worked for AT&T, you were trained to call it an octothorpe. It’s a bit pretentious, honestly. It’s the kind of word people use when they want to sound like the smartest person in the room at a trivia night.
The Rise of the Hashtag
In 2007, a man named Chris Messina walked into the offices of a struggling startup called Twitter. He suggested using the # symbol to group related messages. He called them "channels" initially. The idea was rejected by the founders. They thought it was too "nerdy" for the average user.
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They were wrong.
Users started doing it anyway. By 2009, Twitter officially hyperlinked any word preceded by a #. The name for pound symbol changed overnight for an entire generation. It wasn't a weight marker anymore. It wasn't a phone function. It was a metadata tag.
This changed the way we speak. We started saying the word "hashtag" out loud in actual conversations. "Hashtag blessed." "Hashtag sorry not sorry." It became a linguistic punctuation mark used to convey irony or context. This is the first time in history a technical symbol transitioned from a ledger book to a verbal slang term used by millions of people who have never seen a ledger in their lives.
Professional Variations and Niche Uses
Depending on your industry, you might know this symbol by a completely different set of rules. Context is everything here.
In the world of proofreading and copy editing, the # is a "space" mark. If you see a # scribbled between two words on a physical manuscript, the editor is telling you that you forgot to put a space there. It’s an instruction, not a character.
In music, people often confuse the pound symbol with the "sharp" sign (♯). They aren't the same. The sharp sign has vertical lines that are perfectly straight, while the horizontal lines are slanted. The # symbol is usually the opposite, with slanted vertical lines and straight horizontal ones. If you try to use a # in a formal music score, a conductor might actually throw a baton at you.
In chess notation, the # indicates a checkmate. It’s the end of the game. It’s the most powerful symbol on the board, representing the finality of a win.
The "Number Sign" Confusion
Let’s not forget the most common usage in the United States: the number sign. If you write #1, everyone knows you mean "number one." But this is almost exclusively North American.
In many parts of Europe, the "No." abbreviation (the numero sign) is used instead. This creates a lot of friction in international business. If an American company sends a shipping invoice to a French supplier using the # as a name for pound symbol, there’s a genuine chance of a clerical error.
Interestingly, the US Bureau of Labor Statistics and other government agencies still refer to it primarily as the "number sign" in their internal style guides. They are the last holdouts against the "hashtag" revolution.
Technical Realities in Modern Computing
If you’re a developer, the name doesn't matter as much as the function. In URL structures, the # is a "fragment identifier." It tells the browser to look for a specific part of a page. In CSS, it denotes an ID selector. In Python or Bash scripting, it marks the beginning of a comment—code that the computer ignores but humans read.
- HTML/CSS: Used for hex codes (e.g., #FFFFFF for white).
- Markdown: Used to create headers (just like the ones in this article).
- URL: Separates the main address from an anchor.
- Excel: Appears when a cell is too narrow to show the number. It’s the "dreaded railroad tracks."
The symbol is a workhorse. It’s the Swiss Army knife of the character set. It does the heavy lifting while the more glamorous characters like the ampersand (&) or the at-sign (@) get all the design awards.
Moving Beyond the Identity Crisis
Basically, there is no single "correct" name. Language is democratic. If everyone calls it a hashtag, then for all intents and purposes, it’s a hashtag. But knowing the deep history gives you a bit of a leg up in understanding how information is structured.
If you’re writing for a global audience, the best practice is to be specific. If you mean weight, write "lb." If you mean a number, write "No." or "Number." If you’re talking about social media, "hashtag" is king.
The octothorpe might be the "official" technical name in some circles, but let's be real: no one is going to say "Octothorpe TBT" on a Thursday.
Actionable Takeaways for Using the Symbol
- In Business Writing: Avoid the # for "number" if you have international clients. Use "No." to prevent confusion with currency or hashtags.
- In Web Development: Remember that the # in a URL never gets sent to the server; it’s strictly for the browser.
- In Social Media: Don't overstuff. Two hashtags are better than twenty. The symbol loses its power when it becomes clutter.
- In Design: Use a true "sharp" symbol (♯) for musical contexts and a true "hash" (#) for data contexts. The typography matters.
The next time you look at that little grid on your phone, remember you're looking at a relic of Roman grain shipments and 1960s telephone engineering. It’s a lot of history for four little lines.
Next Steps:
Check your company's internal style guide. Most modern organizations have moved toward "hash" for internal technical docs and "hashtag" for marketing, but you might find an old-school "number sign" rule buried in a 1990s PDF somewhere. Use the most modern term to ensure your SEO metadata doesn't look like it was written in the era of dial-up.