To assist our imagination, which is befuddled by simple words, because, for instance, a door is just a rectangle, but door-ways seldom have any particular shape, so we might hear the word rectangle and end up choosing to stare into the setting sun instead. And doctors don't recommend that.
And then when we look at our guis we might only see partial words like rect, and then we'd have to sympathize with programmers who only get to see that bit of the word.
Triangles are a different kettle of fish though, aren't they?
I, for one, can easily imagine myself with a triangle in each hand.
Now we tell them to get their hypotenuses together. The only thing that remains is equality. Of course, they might've gotten themselves the wrong way round; which might be prevented by construction, or we might just ask someone else to check the corners if they happen to be holding a right triangle themselves.
Containership should now have an obvious meaning in context of two rects; we look at a container of one. Moving this container rect next to the first one, which the two of us created just now, we ask the two to sit nicely together; side by side, or above and below as it suits them best. The rect inside the container is a button.
How's the picture?