You see it every day on TV screens, lumber yards, and smartphone spec sheets. It’s that little double-tick mark sitting right after a number. But honestly, most people are actually typing it wrong. What we call the inches symbol is frequently just a straight quote mark pulled from a standard keyboard, even though typographers will tell you that's technically a "dumb quote."
If you’re looking for the real-deal, professional version, you’re looking for the double prime.
It looks like this: ″.
Most people just hit the key next to the Enter button and call it a day. That gives you ". It works for a quick text or a grocery list, sure. But if you’re designing a poster, writing a technical manual, or just happen to be a bit of a font nerd, there is a massive difference between a quotation mark and the actual symbol for inches.
It's Not a Quote Mark: The Double Prime Explained
The technical name for the inches symbol is the double prime. In the world of geometry and geography, a single prime symbol (′) represents feet or arcminutes, while the double prime (″) represents inches or arcseconds.
The visual difference is subtle but distinct. A standard typewriter-style "straight quote" is perfectly vertical. A "curly quote" (the kind Microsoft Word forces on you) slants or curves to hug the text. The actual double prime, however, is usually slanted slightly to the right but remains a straight wedge shape.
It’s an old-school notation. We’ve been using it for centuries.
Why does this matter? Well, if you use curly quotes for measurements—like writing 6” instead of 6″—it looks amateurish to anyone in the printing or engineering world. Curly quotes are for talking. Primes are for measuring.
How to Actually Type the Inches Symbol
You won't find the true double prime on a standard QWERTY keyboard. It’s buried. Most of us are lazy and use the straight quote ("), and honestly, the world hasn't ended yet. But if you need to be precise, you have to use Alt codes or Unicode.
On a Windows machine, you can hold the Alt key and type 8243 on the number pad. That gives you the double prime. On a Mac, it’s a bit more of a trek through the "Emoji & Symbols" menu (Cmd + Ctrl + Space), where you have to search for "prime."
There’s also the HTML entity method. If you’re a web developer, you should be using ″ in your code. Using standard quotes can sometimes mess with your CMS or create weird formatting glitches when the browser tries to be "smart" and turn them into curly quotes.
The Great Confusion: Inches vs. Seconds
Because the inches symbol and the arcsecond symbol are the same thing, context is everything. If you see $40^\circ 26' 46''$, you aren't looking at forty degrees, twenty-six feet, and forty-six inches. You’re looking at GPS coordinates.
In that context, the symbols represent divisions of a degree. It's a Babylonian system that we just never got rid of.
Interestingly, the United States is one of the few places where the double prime is synonymous with "inches" in daily life. Most of the world has moved on to the metric system, where they just write "cm" or "mm." This makes the double prime a bit of a linguistic relic, tied specifically to the Imperial system and specialized maritime or astronomical navigation.
Why Do We Use Two Marks?
History is weird. The use of the prime symbol likely comes from the Latin primus, meaning first. The foot was the first division. The inch was the second division, or seconda, leading to the double mark.
It’s a shorthand that survived the jump from handwritten manuscripts to the printing press. Early typesetters had limited space. Writing "inches" or even "in." took up valuable real estate on a wooden block. Flicking two tiny metal slivers into the tray was much faster.
Common Mistakes in Digital Documents
The biggest headache today is "Smart Quotes."
You know when you type in a word processor and the straight vertical lines suddenly turn into elegant, curved hooks? That’s the software trying to be helpful. It assumes you are writing dialogue. But if you type a measurement like 12", the software often turns it into 12”, which is technically "12-unclosed-opening-quote."
It looks wrong because it is wrong.
To fix this without memorizing Alt codes, many pros just turn off the "Smart Quotes" feature in their settings when they are working on technical documents. Or, they’ll use the "undo" command (Ctrl+Z) immediately after the software auto-corrects the mark, which usually reverts it to a straight quote. While a straight quote isn't a perfect double prime, it’s much more acceptable for measurements than a curly quote.
Professional Standards and Style Guides
If you look at the NIST (National Institute of Standards and Technology) guidelines, they actually prefer that you don't use the symbol at all in formal writing. They suggest writing out "in" or "inches."
The Chicago Manual of Style and the APA generally agree for formal prose. They want you to use words to avoid any possible confusion. However, in technical drawings, advertising, and tables, the inches symbol is the king of brevity.
In the construction industry, the prime symbols are the law. A blueprint marked with 8' 4" is universally understood. If a carpenter saw 8.33 feet, they’d have to pull out a calculator, and that’s where mistakes happen. The symbol keeps things fast on the job site.
The Screen Size Trap
Marketing has a weird relationship with the inches symbol. Go to any electronics store and look at the tags for TVs. You'll see "55-inch" or "55” Class."
Note that they often use the double quote because it's easier to program into the pricing guns and digital displays. But if you look at the high-end glossy brochures from companies like Apple or Sony, they often take the time to use the actual slanted double prime. It’s a tiny detail that signals "we pay attention to everything."
Practical Takeaways for Your Daily Work
If you want your documents to look sharp, stop using curly quotes for measurements. It’s a dead giveaway of amateur layout work.
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- For casual emails: The straight quote on your keyboard is fine.
- For professional design: Use the true double prime (Alt+8243).
- For web design: Use the HTML entity
″to ensure it renders correctly across all browsers and devices. - The "Cheat": If you can't remember the code, just type the number and "in." It’s cleaner, it’s easier to read, and it’s never wrong.
Getting the inches symbol right isn't just about being a pedant. It's about clarity. In a world where a misplaced decimal point or a confusing symbol can lead to a botched construction project or a misread map, using the right mark actually matters. Check your settings, watch out for "Smart Quotes," and use the double prime when accuracy is the goal.