Behold the Sector

Over the past year I’ve fallen down a traditional furniture design rabbit hole. In the course of this study I discovered a measuring and calculation tool that went out of use until a recent revival: the sector.

Before the 19th century cabinetmakers worked with proportions rather than measurements. When laying out dovetails for a drawer, for instance, rather than using a ruler a traditional cabinetmaker would use a sector and dividers to come up with the spacing. You can figure out this spacing by trial and error but a sector speeds along the process.

To use a sector to divide a line you open the sector up to the length of the line. Then you hold up a divider to the scale etched on the side of the sector to the number you want to divide your original line into. Then you step out the divider on the workplace.

In addition to carpentry the sector was used in navigation, surveying and gunnery. With your handy sector you can also solve multiplication and trigonometry problems. The sector’s invention is attributed to either Galileo Galilei and/or Thomas Hood sometime in the 16th century.

If you’re a teacher or a parent looking for a geometry lesson to do in quarantine Jim Toplin has a free paper sector plan you can download and assemble. I use this paper sector for laying out dovetails. You can also turn a folding ruler into a sector.

With the rising tyranny of inches and feet the sector went out of use in the 19th century. But recently, freaks like me have revived its use and you can buy one for the first time in over a hundred years.

I’ve found thinking in terms of proportions rather than inches revelatory and liberating. And geometry lessons based in practice stick with you much better than those distantly remembered hours of junior high math.

For more information on sectors see The Sector: it’s History, Scales and Uses

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3 Comments

  1. Along these same lines, an easy way to divide a distance into equal spaces ( for example 7) is to place the zero end of a ruler on the top line and slant the ruler until the 7” mark lines up with the bottom line. Tick off the inch marks and you’ll have 7 equally spaced measurements.

  2. I have never heard of this, so I need to study up and it is late and I am sleepy. I can do math with a slide rule.

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