Bevel flattens an area of a T-Spline body by offsetting an edge in both directions.
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Bevel Edge
Bevel Edge adds more control along an edge which allows more control of the body.
The flow of the edge direction can be influenced using Bevel Edge to insert edges from two directions on a body.
Once the target edge is selected, drag the manipulator to set the desired distance of the bevel location. Change the segment count to add more edges.