Splines VS Keys


thewoodwhisperer
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I'm putting this in the Advanced forum because this is purely an academic question. I'm working on a joinery book and there's some terminology I'd like to get right. I know many of us misuse or don't differentiate between these terms, so I'm hoping you guys might be able to look at some of the print resources you have around and also draw on your own knowledge to confirm what I'm about to put into print. 

Because my print resources actually conflict with one another, it's been hard to nail down definitions that work in all scenarios. So here's what I've boiled it down to and it focuses solely on how the joint was made. 

A key is something that is cut and inserted AFTER the miter joint is glued up.

A spline is something that is cut and inserted DURING the miter joint glueup. 

I initially thought the defining factor would be things like grain direction, shape of the piece being inserted, or length of the insert and its relationship to the length of the joint, but all of these things end up having exceptions. So what I'm hoping you guys can do is find a case where the above "definitions" are not correct. 

Thanks for the help.

For reference, here's a key (only goes part-way through the joint and is added after the miter glueup):key.jpg

 

And here's a spline (goes all the way across the joint and is added during the miter glueup):

spline-1.jpg

spline-2.jpg

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Personally, I have always interpreted a "spline" as a loose piece with grain parallel to the main piece, and typically having a straight shape.

A "key", on the other hand, has grain running perpendicular to the main pieces, and typically having an interlocking shape.

In other words, an example of a spline would be a loose 'tongue' used to aid in edge-gluing boards to form a panel.

A key would be like a butterfly inlay used across a crack or joint to prevent opening.

In the case of miter joint reinforcement, under my assumed definitions, they could sort of swing either way, but I choose to call flat ones "splines" and dovetailed ones "keys".

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In motors, spline used to be integral, and keys used to be loose. At some point spline came to be known as an integral key, then spline and key started to be interchanged. Now spline is being called a key, especially an integral key. It is not at all a surprise that this is being discussed, and I doubt you will find consensus. 

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In Tage Frid's book 1 on Joinery,  he refers to both as splines.  But having said that, I did a resent drawer joint that he had shown in his book.  But when researching on the internet the joint name didn't bring up images of the joint the way he cut it.  I think terminology kind of gets tossed around but the trick would be to stay consistent in you referencing in the book

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7 minutes ago, Chet said:

In Tage Frid's book 1 on Joinery,  he refers to both as splines.  But having said that, I did a resent drawer joint that he had shown in his book.  But when researching on the internet the joint name didn't bring up images of the joint the way he cut it.  I think terminology kind of gets tossed around but the trick would be to stay consistent in you referencing in the book

Interesting. I can say that most books I have on joinery, including Tuanton's Complete Illustrated Guide by Rogowski differentiate between them. But it's the notable exceptions that keep me wondering which course I'll take. Thanks for the info on Frid.

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Looking at you pictures a second time I can see were the second one would be more of what is generally referred to as a splined miter in that it goes all the way through and is done in the glue up process, the same as doing a spline miter in a vertical fashion like the corners of a jewelry box or steamer trunk.  Where as the other is done after the glue drys on the miter.  Maybe the term key comes from the fact that it is a shape other then a rectangular prism like a normal spline.  What would that be then a triangular prism?

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Somethings do just get lost in time. Example the proper way to pronounce Sakakawea.

Also that definition is a bit vague. After the project is complete and on display how would you differentiate on method? It is possible to cut the  "key" in but install it during the regular glueup, thus making it a spline and even more confusing.

I feel like more people are going to push back on calling it a key. Also is this a case of more correct. Where we can say a key is a type of spline so calling it a spline is correct but more vague than calling it a key?

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44 minutes ago, Chestnut said:

Somethings do just get lost in time. Example the proper way to pronounce Sakakawea.

Also that definition is a bit vague. After the project is complete and on display how would you differentiate on method? It is possible to cut the  "key" in but install it during the regular glueup, thus making it a spline and even more confusing.

I feel like more people are going to push back on calling it a key. Also is this a case of more correct. Where we can say a key is a type of spline so calling it a spline is correct but more vague than calling it a key?

Maybe that is the ultimate answer, that the book provides working definitions that will be used within itself, while also acknowledging the terms are often used interchangeably.

I've been thinking about this since reading this topic yesterday and really can't come up with a reliable distinction. It gets even harder when I think of how the terms are used in other materials. In concrete form work a 2x4 Keyway is sometimes used to connect a concrete footing to a concrete stem wall, so in that case, the "key" is integral to one of the components, so more of what we would probably call a dado joint. 

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4 hours ago, Chestnut said:

Somethings do just get lost in time. Example the proper way to pronounce Sakakawea.

Also that definition is a bit vague. After the project is complete and on display how would you differentiate on method? It is possible to cut the  "key" in but install it during the regular glueup, thus making it a spline and even more confusing.

I feel like more people are going to push back on calling it a key. Also is this a case of more correct. Where we can say a key is a type of spline so calling it a spline is correct but more vague than calling it a key?

I think you may have touched on the real issue here, with "spline" being the generic term and a "key" being a specific type of spline. So while some may call that simple corner piece that's cut into the frame after glueup a key, it's actually just a specific kind of spline. So then to define a "key" it would be a type of spline that's added after the frame is already glued up. Technically, you can add them during the glueup process but I can't find a single instance where someone does that. Whether it's a corner key or a dovetail key, those are all done after the miter glueup. I feel like we're honing in on it now. 

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On 2/25/2018 at 11:51 AM, Llama said:

Flat piece = spline

"dovetail spline" = key 

+1 for Llama's definition.

I would call both of the joints shown in the OP's images "splined miters".  The fact that one gets cut beforehand for insertion during glue-up and the other gets cut after glue-up doesn't change anything.

For me, a keyed miter would be one in which the splines aren't just flat slips of wood.  Something like this:

5a9475a94a9ea_b00e029b7ae06e103de5783782418f0f--wood-joints-woodworking-projects1.jpg.327bcf236e49fa7fd9a2a9c9cf190c98.jpg

But I've also seen these referred to as "dovetail splines", so that's confusing, too.

One last thought: pretty much every source you'll find for the jig needed to hold a glued-up frame on its corner as it passes over the table saw describes this as a "spline cutting jig".  If someone can point me to a source that describes it as a "key cutting jig", I'll be happy to take a look.

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14 minutes ago, Bombarde16 said:

+1 for Llama's definition.

I would call both of the joints shown in the OP's images "splined miters".  The fact that one gets cut beforehand for insertion during glue-up and the other gets cut after glue-up doesn't change anything.

For me, a keyed miter would be one in which the splines aren't just flat slips of wood.  Something like this:

5a9475a94a9ea_b00e029b7ae06e103de5783782418f0f--wood-joints-woodworking-projects1.jpg.327bcf236e49fa7fd9a2a9c9cf190c98.jpg

But I've also seen these referred to as "dovetail splines", so that's confusing, too.

One last thought: pretty much every source you'll find for the jig needed to hold a glued-up frame on its corner as it passes over the table saw describes this as a "spline cutting jig".  If someone can point me to a source that describes it as a "key cutting jig", I'll be happy to take a look.

If we accept a key as a type of spline than a dovetail spline is as equally correct as calling the flat pieces keys.

Also the above have been covered but what would this one be called?

IMG_4717.jpg?format=original

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