Ask most people what holds a chair together and they will say glue. It is a fair answer and a wrong one. Adhesive is the youngest material in any piece we make, and it is the first to tire. What actually keeps a frame square after four decades of being sat on, dragged across floors and moved between houses is the shape of the joint itself.
Geometry does the work
A mortise and tenon resists racking because the tenon has nowhere to go. Push the frame sideways and the load is carried in compression across the cheeks of the joint, through wood, along the grain — the direction in which timber is strongest. The glue’s job is narrower than it looks: it stops the two halves creeping apart over time. It is a lock, not a load path.
Substitute a butt joint and a generous bead of adhesive and the arithmetic changes completely. Now every newton of that sideways push is carried by the glue line, in shear, across end grain that will drink the adhesive before it cures. The joint may feel solid on the bench. It will not feel solid in 2050.
Wood keeps moving
The second reason is seasonal. Timber takes on moisture in a humid summer and gives it up to central heating in January, and it does so across the grain rather than along it — a wide panel can move several millimetres between June and December. A well-cut joint is designed around that movement: the parts are oriented so their grain runs together, and panels are held in grooves that let them expand rather than pinned flat where they cannot.
An adhesive-dependent joint has no such tolerance. Each seasonal cycle works the bond a fraction, and the failure, when it comes, is sudden — nothing at all for years, then a chair that racks when you sit down.
What this means on the bench
It means the slow parts of the work are the ones that matter. Selecting stock so the grain runs where the load will be. Cutting a shoulder that closes cleanly without being forced. Dry-fitting the whole frame before a drop of adhesive is opened, because a joint that needs a clamp to close is a joint that will spend the rest of its life trying to open.
None of it shows in a photograph. It shows in the fortieth year, which is the only test that has ever counted.