I was in the midst of trimming out a recent remodel when one of the guys described a miter joint he’d noticed while doing the demo work. What he described sounded more like a Japanese temple building joint than the conventional miter joint found in your typical American house. I was intrigued. When he found a sample of the joint and showed it to me, I was amazed.

What we were looking at was a true “lock miter.” (Click images to enlarge)

The leg casing was cut square across the top, then rabbeted at a miter, with a deep dado right against the miter. The head casing was mitered and cut with a corresponding dado that locked right over the leg.

Here in my hand was a miter joint that, although obviously made by machine, was pure elegance. I began to imagine the glorious accolades I would receive if I could reproduce that joint. My mind was swimming with thoughts of fame and glory when it gradually dawned on me that making this joint on site was not going to be easy. The tolerances had to be very tight (indicating a dedicated setup), and I would have to be able to do it for right and left miters (indicating two dedicated setups).

If I was going to solve the mystery of this joint, some research was in order.

The casing had a stamp on the back side that said “CURTIS 1866″. So I did a little digging on this company. My own research, along with some catalog pages provided by Brent Hull, confirmed that what we were dealing with was the Curtis Millworks Mitertite Joint. As it turns out, the Mitertite Joint was only one of several innovations Curtis became known for.

Charles Curtis actually started out in the grocery business. In 1866 he and partner W.G. Hemingway bought a controlling interest in a firm which ran a small door and sash factory. By 1868 Charles and his brother George Curtis had bought a controlling interest in the firm, which became known as Curtis Bros. & Co.

When George joined the team, one of his first ideas was the introduction of factory glazed windows. Previously, window sash was produced without glass, and a builder had to glaze the window on site. Pre-glazed windows were a pretty risky venture, but the gamble sure paid off. Today, if a window came to the job site unglazed we would stare at it in disbelief!

Another interesting innovation can be credited to Judson Carpenter, an uncle who became the company’s purchaser. Judson introduced the idea of grading lumber used in the shop. The same principles he introduced at Curtis Bros. are still used in lumber grading today.

As time went on, Curtis Bros. focused on streamlining their manufacturing practices and standardizing production at each of their factories. Steel gauges and templates were employed to ensure a product’s uniformity regardless of where it was manufactured. This uniformity meant that parts were interchangeable and replacement parts were easy to procure.

All of this progress eventually led to the development of the Curtis Silentite window. According to Brent Hull, the Silentite double hung window represented the “first major improvement in double hung wood windows in 300 years….”

These improvements came courtesy of the Curtis research department created in 1925. This same department also came up with a proprietary chemical treatment to help prevent wood decay. It was here that the Curtis Mitertite interlocking miter joint was born.

As you can see from the drawings below, this joint locks together tightly and requires no glue. The sample I have has no glue in it at all, and it looks great—tight when closed. In fact, the miter joints we found in the house I was remodeling have not opened in 60 years.

	From the top, you can see the dado joint.
	But from the face the miter can’t open, even without glue.
	The rabbet and dado lock the miter together.  The Curtis Mitertite might be the perfect joint for dramatic humidity swings, if we could just figure out a way to cut it that meets OSHA requirements!

I would really like to recreate this joint myself. Unfortunately, I have not been able to find any documentation describing the process used to manufacture it. Curtis was undoubtedly one of the most innovative woodworking companies in history, but either they kept their trade secrets close to the chest, or they have simply been lost in the dustbin of time.

Fortunately for us, this is exactly what a magazine like THISisCarpentry is all about, right?

I’d love to hear suggestions and comments about how this joint could be reproduced on site, or in a modestly equipped shop. Preferably with one setup producing both left and right miters.

So, what are your ideas, fellow carpenters? Can we come up with a way to recreate this joint?

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AUTHOR BIO

Dave Parker has worked in the building trades for most of his career, with a focus on trim carpentry and architectural woodworking. At work he enjoys nothing more than a technically challenging project. At home he enjoys time spent with his family at the beach or in the snow. A graduate of The College of the Redwoods Fine Woodworking program, he currently produces millwork and high end furniture from his shop in southeast Michigan.