Triple-junction velocity compatibility
= Triple-junction velocity compatibility
For a boundary with in-plane normal $n$, an admissible junction <velocity> $U$ obeys $(U-v_{\rm boundary})\cdot n=0$. A ridge uses the mean of adjacent plate <velocities> in the symmetric model, a trench uses the <overriding plate> <velocity>, and a transform uses either adjoining plate <velocity>. Compatibility requires a common intersection of the three velocity-space lines.