This is one of those bindery tricks that folding machine operators might not need often, but when needed can save a lot of setup time. If you’re folding a sheet in thirds and you don’t have a calculator or ruler handy, this technique will mark the sheet precisely in thirds, and you don’t have to convert from decimal to fractions of an inch.
In addition to helping with setup on folders, it’s great when just a few dummies or mock-ups might be needed. Sometimes the bindery guy handles this task and other times it’s an art or pre-press department.
There are a few origami techniques and other ruler and compass techniques that work, but they all end up with the final mark towards the center of the sheet, making it a bit tricky to fold. What I particularly like about this technique is that it marks the sheet at the edge, which is perfect for hand folding your sample.
To do this, you’ll need two sheets of the paper you plan to fold as well as a straight-edge and pencil. A bit of tape might be needed too.
Step One: Carefully fold the sheet exactly in half in one direction. (photo upper left) Then fold it in half in the other direction to locate the center point. Open the sheet so it’s flat.
Step Two: Align the center-mark sheet directly above the second sheet, with the edges lined up perfectly. (two photos at right) It helps to lightly tape the two sheets together to keep them from moving.
Step Three: Use the straight edge to run a line from the center mark of the upper sheet to each corner of the lower sheet. These are the blue lines shown in the photo, lower right. Where the line crosses the lower sheet is your one-third mark (shown at the green arrows.) Make your fold exactly at that point and it will divide the sheet evenly in thirds!
Use care when finding the center point and be sure to locate your straight edge very precisely. If it’s off just a bit, your fold will be off too. Once you get the hang of it you’ll see that it nails the one-third line every time, no matter what size sheet you’re working with.
The reason this works is quite simple. Remember when your seventh grade teacher said you would indeed use geometry and algebra in real life? Well, this is a case in 
point.
If you’re in to mathematics and the proof behind it, you can see it here at the bottom of the page on instructables.com. I’ll spare you the details, but it has to do with the fact that the ratio between sides of a triangle is the same in similar triangles. There. That should be enough to lift the fog from those long lost junior high school memories.
So if you find that the pressman made off with your calculator or ruler, never fear. You now have the wisdom of the ancients on your side.
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