Turns out, rulers aren’t as straightforward as you might expect. While the handy measuring tool dates back thousands of years, the way they’re designed hasn’t always been the same.
In fact, have you ever noticed that the zero mark doesn’t always line up with the ruler’s physical edge? Instead, there’s a small gap before where you’d start measuring. Nope, it’s not a manufacturing flaw. So, why does it exist?
Learn why some rulers have that extra space and how to measure correctly when zero doesn’t start at the edge.
Why Some Rulers Have a Gap Before Zero
The gap at the end of the ruler is used to separate its edge from the point where its measurement scale begins. That gap serves several purposes.
Firstly, it protects the ruler’s accuracy as it ages, ensuring the measuring section remains precise. Because the end, particularly on wood and plastic rulers, is vulnerable to chips, dents, and other types of wear, a consistent starting point is critical to measure accurately. Moving the measuring starting slightly inward keeps it protected.
Also, the gap gives manufacturers room at the end of the ruler without making the cut or molded edge part of the calibrated scale. Small variations at the outer edge won’t affect the location of zero or the spacing between the markings.
The gap also provides a useful visual cue, especially for children learning how rulers work. Stanford’s DREME Teaching Resources says that understanding the starting point plays an important role in children’s learning of measurement.
Why doesn’t every ruler have a gap?
Rulers with an edge-to-edge scale aren’t inherently unreliable. Some precision rules place graduations at or near the end, while others have specialized scales around zero. Some also come with a keyhole. So if you’re ruler shopping and see a few different options, don’t be surprised.
How to Measure With a Ruler That Starts After the Edge
One downside to this design is that you won’t be able to start measuring by lining your ruler up with the edge of the object. Instead, the measurement will have to start at the zero line rather than at the ruler’s physical end to get an accurate reading.
When measuring, place one end of the object directly against zero, keep the ruler straight alongside it, and read the mark where the opposite end lands. The measurement comes from the distance between those two points, not from the ruler’s physical edge.
Sources
- Stanford DREME Teaching Resources, “The Mathematics of Measurement”
- Empire Level, “Rulers”
- L.S. Starrett Company, “Precision Rules, Straight Edges & Parallels”
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