The Trunnel

A trunnel — treenail, pronounced "trunnel" — is a wooden dowel used to fasten planking to frames in traditional boatbuilding. Oak trunnels are driven dry into pre-bored holes; when the boat is launched, the wood swells in seawater, tightening the joint. Unlike iron fasteners, trunnels do not corrode in salt water. Unlike bronze bolts, they create no galvanic reaction with the surrounding timber. The trunnel is the same material as the structure it holds together. Wood fastens wood.

Bone pins in orthopedic surgery work on the same principle of material compatibility. When a fracture must be stabilized, metal plates and screws are standard. But metal creates stress shielding — the bone beneath the plate carries less load, weakens, and may fracture again when the hardware is removed. Bioabsorbable pins made from polylactic acid or similar polymers hold the fracture during healing and then dissolve, leaving the bone to carry its own load. The fastener matches the material it serves, and when its work is done, it disappears.

In software, convention over configuration is a trunnel in code architecture. Instead of external configuration files (the metal bolt) telling the framework where to find models, views, and controllers, the framework expects them in predefined directory locations with predefined naming patterns. The code is organized by the same logic that the framework uses to find it. The convention is made of the same material as the project — directory structure and naming — rather than a separate configuration language. No impedance mismatch between the fastener and the frame.

The trunnel is the principle that the best fastener is one made of the same material as the structure it joins. Metal in wood creates differential expansion, corrosion, and galvanic decay. Foreign configuration in code creates maintenance burden, synchronization failures, and cognitive overhead. The trunnel avoids these problems by being indistinguishable from its surroundings — it swells when the planking swells, shrinks when the planking shrinks, ages at the same rate, and responds to the same environment. The fastener that matches the material disappears into the structure. The structure holds itself.