Rule of Thirds Gauge

We're all familiar with the rule of thirds, and how we can use it when composing a shot or in the darkroom to elevate a photograph to a masterpiece. I've been finding it very tedious having to measure thirds in the darkroom with a ruler, or cutting appropriately-sized pieces of paper with lines drawn to mark the thirds. I've found an elegant solution to this problem that is both easy to DIY and is adaptable to all sizes of paper.

Inspired by the golden ratio gauges often constructed and used by woodworkers, I took pen to paper to conceptualize what a similar gauge would look like when constructed to give three equal sections. Here are my initial scribblings:

A quick sketch lays out the unit lengths for each arm of
the gauge. Three units for each long arm, two units for
the short arms.

The idea was simple enough, and to my surprise, it was even easier to construct a functional prototype. I sort of brute-forced the solution with super simple math, working with a unit length of 9cm. I'm sure there is a mathematical proof that shows why this device works, but I'm happy enough having sorted it out so simply. The two long arms are 27mm from the pivot point to the tip, and the two short arms are 18mm from their pivots to the tip. All pivot points are 9cm apart. These lengths can of course be adjusted for any sized gauge you wish to construct, as long as the ratios stay the same. Here is a photo of my prototype, made of cardboard and complete with toothpicks to hold everything together:

The gauge functions by first adjusting the two outer points to the desired dimension. When set, the two inner points will show the exact lines that divide the paper into three sections. Easy thirds, without a ruler!

For the final product, I took some black matboard and cut it into 1" strips. I drilled holes with the four pieces clamped together so the holes would be exactly the same on all pieces. The ends were cut to points, and brass brads were used to hold the whole contraption together.

Simplicity, ease of construction, and versatility, this gauge has it all. So far it's proven to be extremely valuable in the darkroom, and I hope to see others start using this tool as well!