What exactly is “width” in geometry?
I had never thought I would ever wonder what “width” is until my 12-year old daughter came home one day and told me that her math teachers (not just one but two) told her that “width” in geometry is the vertical side of a rectangle. That to me was like saying up is down and down is up. How could this be?
It turns out that her teachers are not alone. Take a look at this page I came across while Googling on the topic. It says:
In the case of a square or a rectangle, the expression length (1) is commonly used instead of base and width (w) instead of height. In the case of a circle the expression diametre (d) is used.
“Width instead of height” is very much like saying up is down. Where did this come from?
And, what is even more disconcerting is that the teachers are not aware of this ambiguous nature of “width.” Two other teachers told me that width is always the horizontal side, and another told me that she has heard others call the vertical side “width.” So, apparently, each is teaching their students in an authoritative manner their own definition of “width.” If a 12-year old is told by her math teacher that “width” in geometry refers to the vertical side, why should she doubt it? Unless, of course, she was also told that “width” can be vertical or horizontal (i.e. direction neutral, like “length”). But that is apparently not what is happening at school in New York City.
If we teach our kids that a triangle has three sides, we don’t want them thinking that the definition of “triangle” could be flexible as if it could have four or five sides. If the definition of “width” is not universal, they need to know that. Here is a case in point. The following question was in 2016 Common Core Math Test for Grade 6.
“A carpenter built three bookcases, A, B, and C, to stand next to each other along a wall. The total length of the wall is 456 centimeters. The carpenter will build two more bookcases, D and E, along the same wall. These two bookcases will have equal widths. The widths of bookcases A, B, and C are shown in the table blow.”
Now, if you were taught that in geometry, “width” means the vertical side and “length” means the horizontal side, you would have to be confused reading this question. The correct response would be, “But why should the ‘widths’ of the bookcases have anything to do with the ‘length’ of the wall in fitting the bookcases?”
My daughter tried to figure this out by drawing a bunch of bookshelves with different heights, and eventually gave up because the question made no sense to her.
In Merriam-Webster, the word “width” has no ambiguity: “the horizontal measurement taken at right angles to the length.” After all, the word “height” has no ambiguity, so why should we think “width” would?
But looking at the Oxford dictionary gives us a slightly different answer: “The measurement or extent of something from side to side; the lesser of two or the least of three dimensions of a body.”
In other words, in a rectangle, “the lesser of the two” sides would be called “width” regardless of the orientation. So, it appears that this is an American-versus-British issue, or is it? (Note the spelling of “diametre” on the page from fao.org quoted above.)