Prime marks are not quote marks
Feet and inches are written with the prime and double prime: 5′ 6″. The characters on the keyboard, ' and ", are the ASCII apostrophe and the ASCII quotation mark, and they are not the same thing. They are close enough in most fonts that nobody notices on screen, and far enough apart that a typesetter, a CAD import or an engineering document will notice immediately. The prime is upright; the apostrophe in a proportional font is usually curved or slanted.
The same distinction runs through angles and time. Latitude is written 40′ 26″ with primes for arcminutes and arcseconds, never with quote marks. And when you are actually quoting somebody, you want the curly quotation marks “ ” rather than either of the above. Three visually similar pairs, three different jobs, and only one pair on the keyboard.
The dash you want is almost never the hyphen
English uses four horizontal strokes that look alike at small sizes and mean different things.
| Character | Name | Used for | Example |
|---|---|---|---|
| - | Hyphen | Joining words, breaking lines | cast-iron skillet |
| – | En dash | Ranges, spans, connections | pages 12–18, 2019–2024 |
| — | Em dash | A break in a sentence | the pour — all of it — failed |
| − | Minus sign | Arithmetic and negative numbers | −40° |
The minus sign is the one people miss most often. A hyphen used as a minus is narrower than the plus sign it should balance, so a column of figures containing negatives looks visibly ragged. In a spreadsheet cell the true minus is worse than useless because it will not parse as a number, so use it in prose and headings, not in data.
Degree, micro, and the characters with lookalikes
The degree sign ° is not a superscript zero, not a masculine ordinal indicator º, and not a ring above. On a page all four are a small circle; in a search box or a database sort they are four different characters. Temperature can be written as °F with a separate degree sign and letter, which is the recommended form, or with the single precomposed character ℉, which some older systems render as a box.
The micro sign µ and the Greek letter mu μ are separate code points that most fonts draw identically. Unicode kept both for round-trip compatibility with older encodings. Either will be read correctly by a person; if the text is going into a system that compares strings, pick one and stay with it. The same problem exists for the ohm sign Ω against the Greek capital omega Ω, and for the Kelvin sign K against a plain capital K.
Fractions that are one character, and fractions that are not
Unicode has precomposed fractions for the common cases: halves, thirds, quarters, fifths, sixths, sevenths, eighths, ninths and tenths. Anything outside that list has to be built from a superscript numerator, the fraction slash ⁄, and a subscript denominator — for example 5⁄16 assembled by hand. Some fonts turn that sequence into a proper stacked fraction automatically; most do not, and you get a diagonal fraction that is still perfectly readable.
Precomposed fractions have one practical drawback worth knowing about in shop work: they do not sort or calculate. A cut list containing ½ is text, not a number, and no spreadsheet will add it up. For anything that has to be arithmetic later, write 0.5 or 1/2 and keep the pretty version for the printed sheet.
Where these characters break
Every character here is standard Unicode, and every modern browser, phone and word processor renders them. What breaks is older and narrower systems: fixed-width form fields that only accept ASCII, plain-text email gateways, legacy point-of-sale printers, embroidery and CNC controllers, and any database column defined as Latin-1 rather than UTF-8. If a character lands in one of those and comes out as a question mark, a box, or the notorious  pair, the character was fine and the encoding was not. Test with the actual destination before you commit to a fancy dash in a product name.
Questions people ask
What is the difference between an en dash and an em dash?
Width and job. The en dash is roughly the width of the letter n and marks a range or a connection: pages 12–18, the New York–Boston line. The em dash is roughly the width of an m and marks a break in a sentence, standing in for parentheses or a colon. American style usually sets the em dash tight against the words on either side; British style usually uses spaces. A hyphen is shorter than both and joins words rather than separating clauses.
How do I type feet and inches correctly?
Use the prime U+2032 for feet and the double prime U+2033 for inches, so five foot six is 5′ 6″. The apostrophe and quote characters on the keyboard are different code points and, in most proportional fonts, visibly curved rather than upright. In casual writing nobody will object to the keyboard characters, but in drawings, specifications and anything typeset properly, the primes are the correct marks.
Why does a character show as a box or a question mark somewhere else?
Two different failures look similar. An empty rectangle means the font has no glyph for that character; the data is intact and a different font will show it. A question mark or garbled letters means the text was converted through an encoding that cannot represent the character, usually a Latin-1 database column or a plain-text email gateway, and at that point the original character is gone. If a symbol has to survive an unknown system, spell it out: deg, plus/minus, 1/2.
Do I need to install anything for these to work?
No. These are ordinary Unicode characters and everything happens in this page. Clicking copies the character to your clipboard, and you paste it wherever you were going. What the character looks like after that depends on the font in the destination, and a handful of the rarer symbols here will fall back to a different font, which can make them look slightly out of place beside the surrounding text.