direction of number lines

[Anonymous]

The example below (from 6.NS.7a) emphasizes the direction of the number line. I think, most people assume that the horizontal line points left and the vertical line points up, though it does not have to be that way and is not is some applications. Is the expectation that stuents always should specify the direction of the number line before using terms like “left” and “above”?

 
For example, interpret –3 > –7 as a statement that –3 is located to the right of –7 on
a number line oriented from left to right.

 

[Bill McCallum]

Did you mean to say “horizontal line points right”? Either way, I’m not sure about using the terminology “points right” and “points left”. A number line has arrows at both ends, one pointing in the positive direction and the other pointing in the negative direction. The term “oriented from left to right” was meant simply to indicated that numbers on the right are greater than numbers on the left. This should be a standard convention with number lines, and it should also be a convention that a number line is horizontal; students don’t need to say this every time. Once students start working with coordinates, they have a vertical axis as well as a horizontal axis. I would resist calling these number lines, although obviously they are descended from number lines; rather I would call them axes. And, of course, there is a convention about vertical axes as well, that numbers above are larger than numbers below.

[Author]

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