Frame joints: How are picture frames joined?
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Frame joints: How are picture frames joined?

Framing Services / Frame joints: How are picture frames joined?

Have you ever wondered how the different parts of a picture frame are joined together? If so, or if you’d just like to learn more about picture frame joints generally, we’ve got the answers.

Here at Soho Frames, we like to think of ourselves as specialist carpenters — in the sense that we mainly join rectangular frames. That said, we also make circular and oval picture frames, as well as other “exotic” shapes like hearts, hexagons, triangles and various irregular shaped frames to suit particular artworks. There are very few limitations to what we can do.

When it comes to joining a picture frame together, there are several approaches that framers like ourselves can take, and not all of these are obvious. The meticulous attention to detail and the beauty of a finished work of art isn’t always what can be seen, but rather sometimes what can’t be!

With all that in mind, here are the main approaches used to put together frame joints at 90 degrees.


With underpinning, the frame lengths are joined on a special machine that drives a v-shaped pin into the underside of the frame. It’s an efficient way of joining, and can be used on both hard and soft woods.

Of course, although the pins are specifically designed to pull the two ends of the wood together, we look at them as clamping the wood while the glue dries. The glue forms a much tighter bond than the actual wood fibres themselves, which can otherwise shrink away from the pins over time. This would create a gap on the front side, something that can lead to the frame literally falling apart. Ultimately, glue keeps everything together.

Although a brilliant system for many applications, underpinning does have its limitations. If the frame is very deep (over 30mm) or made from a hard wood like oak or ash, a different method is required.


For harder woods, we use a special routing machine that mills a wedge-shaped hole into each end of the picture frame lengths. Frames with joining angles other than 90 degrees and with depths up to 80mm can also be efficiently joined with this method.

Once the milling is finished, the pieces are pulled together into a tight join by pressing in a special butterfly-shaped plug into the cavity. Plugs come in different sizes, and deeper frames will require the plugs to be stacked, all of course bonded with glue.

Finally, the underside of the frame is sanded flush should the final plug protrude a little.


Splines are small bits of wood that are inserted in a mitre to reinforce the joint. They can be either made of the same wood as the picture frame, or from a different wood to highlight the join.

When splines are used to join a picture frame, the frame lengths are held in a special custom made jig, with the spline cavities milled and the splines themselves inserted and glued into place. Once the glue is dried, the splines are sanded flush with the surfaces of the frame corner, before the entire frame is then finished. This could mean staining and polishing or for a natural wood look, polishing only to various degrees of shine.

Spline joints are very strong and we often use these when custom spraying, as it is the best method to prevent tiny hairline stress cracks appearing in the front surface joint. How do we do this? Well, we shift the uppermost spline right up so that it actually forms the top face of the frame. This means that, as well as holding the corner together, it also covers the join from the two pieces. Not a bad tip for any other framers reading this article!

The unique SohoFrames approach

Here at SohoFrames we also design, create, and build picture frames for worldwide clients looking for something totally unique, based on their own brief, vision or idea, which we then flesh out.

Take our Islamic art inspired registered designs, for example, which are particularly complicated to build. Corners are often a problem here, as it’s difficult to predict how the design copes with the change of direction. The usual 45 degree edge at the end of the frame length can look out of place, spoiling the way the eye travels around the artwork. By artwork, we are talking about the frame design we made for this article to demonstrate the concept. In this sample chevron, we designed the corner joint to exactly match the carved top layer. Being a tight interference fit, the picture frame joint is extremely strong.

So there you have it: frame joints explained. Learn more about our framing services here, and don’t hesitate to contact us to ask a question or to make an enquiry.

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