Subdiv Surfaces > Full Crease Edge/Vertex
Full Crease Edge/Vertex modifies the appearance of a subdivision surface by applying an edge shaped feature along the selected region of the mesh.
Applying a crease is useful when you want a sharp edge or transition on a surface. You can also apply a partial crease, or uncrease previously creased regions of the surface. The feature produced by a full crease appears more pronounced on the surface than a partial crease.