Archive for June 6th, 2008

Is a square a rectangle?

June 6, 2008

I like this question. My first reaction — since I get this pretty much every semester that I teach a problem-solving or geometry class — is to ask what the definition of a rectangle is. Most people respond that it’s a quadrilateral with 4 right angles, maybe they add something about the opposite sides being parallel and/or equal, and then I ask if a square fits that definition. They answer yes, and the problem is solved.

But I think the question is really a little more subtle than that. In all the children’s books that we’ve acquired on shapes, none of them show a square on the rectangle page. Years of reinforcement that squares and rectangles are different shapes is hard to overcome with a single definition.

Furthermore, when I started teaching Geometry I learned that 2300 years ago Euclid didn’t define rectangles (which he called oblongs) in quite the same way as we do. Here’s a page from Oliver Byrne’s 1847 translation of Euclid’s Elements, which is one of my favorites because Byrne sure liked his color markers. He uses oblong the way we use rectangle.

Notice that Euclid said that an oblong did not have all four fides equal: a fquare was a completely different beast, not a special kind of rectangle. Euclid kept this distinction with all his geometric figures: a rhombus couldn’t have right angles (so a square wasn’t a special kind of rhombus either), a parallelogram (rhomboid) did not have right angles or equal sides, and an isosceles triangle had exactly two equal sides, not at least two. At Euclid’s Geometric Figures party when the figures divide into teams, the squares knew EXACTLY where to go, and it wasn’t with the rectangles: it was a partition, rather than a Venn diagram.

Another place where geometric problems can occur is with triangles. I think of the stereotypical triangle [in the US — is it true in other countries as well?] as being one with a horizontal base, and probably isosceles.

But, just like the definition of rectangle, that hasn’t always been the case. In in “Words and Pictures: New Light on Plimpton 322”, Eleanor Robson explains, “if we look at triangles drawn on ancient cuneiform tablets like Plimpton 322, we see that they all point right and are much longer than they are tall: very like a cuneiform wedge in fact.”

Neither triangle is better or worse than the other, but they are different, illustrating the cultural influence on mental images of shapes. I find that intriguing.

I believe that the page of Byrne’s translation is fair to include because its over 70 years old. And an edition only sold for $300 in the ’70s — can you believe it? Not that I had more than $5 at any one time in that decade, but still, if I had and I wasn’t buying dollhouse furniture, I’m sure I would have bought it.