Year has just started, brain might not be working yet. What am I missing here? I modify the subd (subd has reflect along x-axis) and then geometry seems to pop out elsewhere?
SUBD.3dm (75.2 KB)
Hi @ftzuk
You are pulling out not just the edge, but also the two vertices attached to that edge. In order to maintain the symmetry, Rhino adds additional faces. If you want to avoid it, in this case either move just the outer vertex or move the edge and vertices only vertically. Hope it makes sense 
-Jakob
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