You've seen it a thousand times. You’re looking at a laptop screen size or checking the dimensions of a new bookshelf, and there it is: two little vertical lines perched next to a number. Most people call it a double quote. Some call it "ditto marks." But if you want to be technically accurate, what is the inch symbol really? Honestly, it’s one of those things that seems incredibly simple until you start digging into the world of typography and international standards. Then it gets messy.
Most of us just hit the key next to the Enter button. It works. People get what you mean. But if you’re a designer, a machinist, or just someone who hates being wrong, that little mark is actually called a double prime.
The difference between a "curly" quote and a true inch symbol is the hill that typographers are willing to die on. It’s not just about aesthetics. It’s about history.
Why Your Keyboard Is Actually Lying To You
Look at your keyboard. Find the key with the apostrophe and the quotation marks. When you type that in a Word document or a Google Doc, the software usually "fixes" it for you. It turns those straight lines into curly, elegant shapes that point toward the text they’re hugging. These are known as "smart quotes."
They are beautiful for dialogue in a novel. They are absolutely wrong for measurements.
The actual inch symbol—the double prime ($''$)—is a slanted, straight mark. It doesn’t curve. It doesn't care about the text around it. It is a mathematical notation. In the world of Unicode, the standard that tells computers how to display characters, the double prime is located at U+2033. The standard double quote you use for "Hello" is U+0022. They are cousins, sure, but they aren't the same person.
Back in the days of manual typewriters, space on the keyboard was at a premium. Manufacturers didn't want to waste a whole key on a specific mathematical prime and another on a quotation mark. So, they compromised. They created the "neutral" or "vertical" quote. It was a one-size-fits-all solution. We’ve been living with that compromise ever since, even though our modern screens can handle thousands of distinct characters.
Prime vs. Quote: The High Stakes of Accuracy
You might think I’m being pedantic. Maybe. But in technical fields, clarity saves money.
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Think about a blueprint. If a machinist sees a mark that looks like a stray pen flick because the font used "smart quotes" instead of a proper inch symbol, things can go sideways. The prime symbol ($'$) denotes feet, and the double prime ($''$) denotes inches. This system stems from the Latin primus (first) and secundus (second) subdivisions of the foot.
Basically, the foot is the first division, and the inch is the second.
Real World Examples of the Inch Symbol
If you’re writing out the specs for a 65-inch television, the "quick and dirty" way is 65".
The professional, technically correct way is 65″.
Notice the slight slant in the second one? It looks more intentional. It looks like it belongs in a manual for a Boeing 747.
Interestingly, the maritime world uses these symbols for coordinates too. When you’re looking at GPS data, you’ll see degrees, minutes (prime), and seconds (double prime). If you’re at 34° 03' 08'', that last bit is the "seconds," but the symbol is identical to the inch symbol. If you use a curly quote there, you look like an amateur to any navigator or cartographer.
How to Actually Type the Real Thing
Since the keyboard doesn't have a dedicated "prime" key, most of us just settle. But you don't have to. You've got options depending on how much you care about perfection.
If you are on a Mac, you can pull up the Character Viewer (Control + Command + Space) and search for "prime." It’s a bit of a trek.
On Windows, it’s even more of a chore. You’d use the Alt code. Hold down the Alt key and type 8243 on the number pad. Who has time for that? Nobody. That’s why most of us just stick with the straight quote. But if you’re using professional software like Adobe InDesign, there’s usually a toggle to turn off smart quotes specifically for measurements, or a Glyphs panel where you can double-click the true double prime.
The CSS and HTML Trick
For the web developers out there, don't use the quote key.
Use the HTML entity ″ (with a capital P).
′gives you the foot symbol: $′$″gives you the inch symbol: $″$
It makes a difference in how screen readers interpret the text. Accessibility is a big deal in 2026, and using the right semantic mark helps tools for the visually impaired understand that they are reading a measurement, not a quote from a person.
The International Confusion
Wait, it gets more complicated. If you go to France or Germany, they don’t use our quotation marks anyway. They use guillemets (« »). But guess what? They still use the double prime for inches and seconds. It is a universal mathematical language that transcends the local "flavor" of punctuation.
However, most of the world has moved to the metric system. For them, the inch symbol is a relic of the past, something seen mostly on imported American products or in specialized industries like plumbing (pipe diameters are still weirdly imperial almost everywhere). In those contexts, the "inch" is often abbreviated as "in" to avoid the symbol confusion entirely.
Honestly, "in" is the safest bet if you’re worried about your font not supporting the Unicode prime character. Some cheap fonts don't even include the glyph for U+2033. If the font doesn't have it, your beautiful inch symbol turns into a "tofu" block—one of those annoying little rectangles with an X in it. Not a good look for your website.
Why Does This Even Matter?
You might be wondering why I’m spending 1,500 words on a tiny tick mark.
It’s about the "uncanny valley" of professionalism. You know when you look at a flyer or a website and it just looks... off? You can't put your finger on it, but it feels cheap? Often, it’s the typography. It’s using a hyphen instead of an em-dash. It’s using "straight" quotes for dialogue. Or, in this case, using "curly" quotes for a 12-inch sub sandwich.
The double prime is a signal. It tells the reader that the person who wrote this knows their stuff. It’s the difference between a DIY blog and a technical journal.
Misconceptions That Need to Die
Let’s clear some stuff up.
First, the inch symbol is NOT the same as the ditto mark. They look identical in some fonts, but the ditto mark is used to indicate that the text above it is repeated. Using an inch symbol as a ditto mark is technically a crime against grammar, though a minor one.
Second, the symbol doesn't go before the number. It’s not like a dollar sign. I’ve seen people write $''12$ and it makes my brain hurt. It’s always $12''$.
Third, space or no space?
In most style guides, there is no space between the number and the inch symbol. You write $10''$, not $10\ ''$. However, if you are using the abbreviation, some guides prefer a space: $10\ in$. Just pick one and be consistent. Consistency is more important than almost anything else in writing.
What to Do Moving Forward
If you’re a regular person just sending an email, keep using the quote key. Don't be a weirdo. No one expects you to memorize Alt codes for a casual message about a 5" x 7" photo print.
But if you are building a brand, writing a book, or designing a product interface, do it right.
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Your Action Plan for Accuracy:
- Check your software settings. In Google Docs or Microsoft Word, go to Tools > Preferences or Options > Proofing. Look for "Smart Quotes." If you’re writing a lot of technical specs, turn it off. If you’re writing a novel, keep it on.
- Use the HTML entity. If you’re posting to a blog or a CMS, use
″. It’s cleaner and better for SEO and accessibility. - Audit your "About" page. If you’re a business that lists dimensions, check how they look. Are the quotes curling toward the numbers like they’re trying to eat them? Fix it.
- Pick a side. Decide now if you’re a "symbol person" or an "abbreviation person." If you choose symbols, use the true double prime. If you choose abbreviations, use "in." for everything. Just don't mix and match $12''$ and $12\ in.$ in the same paragraph. It looks messy.
The world of punctuation is deeper than most people realize. The inch symbol is a perfect example of how a tiny detail carries a lot of historical and technical weight. Now that you know the difference between a quote and a prime, you’ll never be able to un-see the mistakes on every menu and billboard you walk past. Sorry about that. But at least your own work will be flawless.
Start by looking at your most recent project. Highlight those dimensions. If those marks are curving, hit backspace and find the "straight" version. Your typography—and your readers—will thank you for the clarity.