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I'm having a "challenge" in my geometry classes getting students to avoid using rulers as measuring devices in constructions. As natural as that usage is, they're only supposed to use them to connect points to form line segments and extend line segments. It's very understandable when they use them to mark off a line segment congruent to a given line segment. But then they extend the idea to a sort of "guess and check" when they really should be using a compass instead. At the same time, I don't want to criticize their ingenuity or suggest that tools shouldn't be used as fully as possible to solve problems when appropriate.

So what I'm leaning towards now is having 25 or so unmarked straightedges to have students use, so they are constrained to using them as Euclid intended. I haven't ever seen them sold as such, so I'm wondering if people have any suggestions about common "blank ruler-like" products that could be used as substitutes. (I would like to have an actual product for them to use, and not, say, using the edge of another textbook or an unsharpened pencil or something like that.) Alternatively, I might get something like a simple piece of moulding cut into foot-long segments and sanded down.

Does anyone have any experience or suggestions along these lines?

For instance, here's an example of a brilliant but illegal construction I'd like to avoid seeing:

Here, we bisect $angle BAC$ by swinging an arc with $A$ at the center that intersects the angle at $D$ and $E$. Then, using the ruler to measure $DE$, construct $F$ as the midpoint. Then $overlineAF$ is our angle bisector.

ETA: I visited my local craft store and found 10" craft sticks in a pack of 50 for $7. The edges are rounded like popsicle sticks, but they're long enough that students will be able to draw any line segments they want with just the straight part. I think this is going to work really well for me. Thanks to everyone for the suggestions!

ETA after the constructions unit: I'm really glad I steered away from the rulers. For instance, when we discussed as a class how to determine whether every point on the perpendicular bisector of a line segment was the same distance from the two endpoints of the line segment, I asked "What tool do we use in constructions to verify that two line segments are the same length?" and everyone held up their compasses. That surely wouldn't have happened if they had a ruler on their desk.

A few downsides along the way. First, not being lacquered, the craft sticks got written on. Sometimes, students would play with them in ways that made them more splintery than I'd like to have classroom supplies. Finally, and most surprising, both the students and I were constructing clearly parallel lines that were clearly skew to the degree that I'm not sure they're as uniformly straight as you would think. Not a huge deal, because I was verifying their work with a compass anyways, but it was a bit of a bummer.

I have lots of ideas, which, it turns out, aren't going to fit into a comment.

• If you don't need something too long, but want something that is pretty uniform and consistent, popsicle sticks (also sold as craft sticks) might work. The are pretty durable, but relatively cheap and, ultimately, disposable.




• Others^[1] have suggested paint stirrers (which you are often given gratis at hardware stores when you are buying other things). I originally dismissed this idea, as they tend to be fairly rough cut and made from soft pine, which may not hold an edge for very long. However, some time with a belt sander (and possibly the application of some wood lacquer) might make this a very viable option.




• Wood molding or other small pieces of wood can be cut down to size. I would suggest that the best strategy here is to go to the hardware / lumber store and see what they have. You might find something you like. This might be a little more expensive, but you can get the pieces cut down to whatever length you like (or do it yourself if you have the tools). If you go this route, you should probably use a harder wood (such as oak); it will hold an edge longer.




• Going very low tech, take a piece of $8frac12 times 11$ inch printer paper, and fold it in half the long way two or three times (therefore obtaining a piece of paper which is $11$ inches long and one or two inches wide. If you put a sharp crease into the paper, it should last for an hour or two.




• Pieces of cardboard, paperboard, or cardstock. Cardstock (which is, basically, a subset of paperboard) can be obtained for cheap from most office supply stores, or can be cut out of cereal boxes (take a cereal box, a good ruler, and (preferably) an X-acto knife, and cut the box into one inch wide pieces). Larger pieces can be cut down to size with a paper cutter or X-acto knife. If you need something more durable, cut several pieces to size, then glue them together.




• If you are wealthy, there is always this guy. Or this. That's what grants are for, right?

[1] Tip o' the hat to André 3000 and Darrel Hoffman.

A cheap Venetian blind with thin plastic slats can be cut up with ordinary scissors and will yield many straightedges of any length you choose. The slight bow in the blade will flatten out when pressed against paper, and makes it easier for fumble-fingered people like me to pick up the straightedge from a flat surface.

Try shopping for a "paint guide".

These products have:

• a straight plastic or metal edge


• a grippable portion that is not on the edge


• no ruler-like markings


• low prices.

Drafting triangles also have unmarked edges that are intended to be drawn along, but their accurate angles are too useful in your intended application.