Make a Sector from an Old Folding Rule

by Patrick Harper - Blood, Sweat, and Sawdust


The Sector

How do you quickly divide an interior space of 17 5/8 inches into three equal parts?  With a sector.  Furniture builders and architects have reached for the sector often to quickly divide spaces.  Unfortunately, it’s a tool that has been largely forgotten in today’s machine driven world.

The sector is nothing more than a pair of folding arms used to create a series of proportional triangles.  To divide a space, line up the sector with one of the markings that is a multiple of the division you want.  For example, if you want to divide the space into four equal parts, you could use the four, eight, twelve…you get the point.  Then, just set a pair of dividers to the division you want.  If you selected twelve, align your dividers with the three.  If this isn’t entirely clear, don’t worry.  I will write another post detailing their use.


To make the sector, start by marking the center point of your hinge, where the inside of each arm intersect.  I used an accurate straightedge to carry a line through with a pencil.  It’s critical that this is accurate.


Then, walk your increments off with a pair of dividers.  You may have to use some trial an error here.  Twelve increments works great, due to the amount of whole number ratios you can get.  Unfortunately, my first step would have still be in the brass, so I went with eight for this one.


With the increments stepped off, I carefully carried a line across both arms with a marking knife.

Completed Sector

From there, it was just a matter of filling in the lines for visibility and numbering my graduations.  Stay tuned for part two, where I will demonstrate the sector in use.

Click here for part 2 in this series: How to use a sector