“What’s the line in a fraction called?”
While we were at lunch last week, The TenYearOld’s mind turned to fractions, as fifth grade minds are prone to do. She had learned in school that the top number in a fraction is called a numerator, while the bottom number is the denominator, but the line in the middle didn’t seem to have a special name.
That was disappointing.
“Mommyo,” she asked between bites of grilled cheese.
“What’s the line in a fraction called?”
This was easy.
“Division bar,” I said.
“Fraction bar,” Daddyo said at the same time.
Gran chimed in with something that started with v, but I didn’t catch it.
So I decided to look it up. Turns out that little line has lots of names. People frequently refer to it as the:

 division bar
 fraction bar
 vinculum
What’s a vinculum?
I had never heard of vinculums before, so I did a little research on them. MerriamWebster defines a vinculum as
a straight horizontal mark placed over two or more members of a compound mathematical expression and equivalent to parentheses or brackets about them
Put more simply, a vinculum is a horizontal line placed over a group of math terms to show that they are related to one another.
Even simplified that definition sounds really broad, and like vinculums might appear in lots of other places besides fractions.
Where else can you find vinculums?
Mathematicians use vinculums to designate:

 the radicals in roots
 repeating decimals
 line segments
 the radicals in roots
Vinculums are also apparently used in complex conjugates and to negate logical statements, but I’m not prepared explain either of those situations to The TenYearOld, so I’m going to stop here.
Related Links:
 Vinculum (Wolfram Math World)
4 Responses to ““What’s the line in a fraction called?””
Don’t tell the kid this, but math sucks.
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Sadly, don’t need to tell my kid. She has a love/hate relationship with math already. I’m employing heavy doses of Penrose the Mathematical Cat to try to turn things around. She doesn’t care much for the math being taught in her school right now, but the Penrose books are great reminders that math can be more interesting.
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For a lot of kids, it needs to be taught individually and visually. That’s almost never done.
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Really hard to do in a public school setting, which is why I try not to ignore math at home. Even if all I’m doing is pointing out that math is why we are able to figure out the best bargain on cereal at the grocery store or assess the value of whatever bit of clothing we are thinking about buying, I hope it’s reinforcing that math is everywhere, and having some basic proficiency in it matters.
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