SketchUp Rectangle Tool: Face entities
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Face entities are flat plane-like entities that combine to form the 3D geometry in a SketchUp model. Faces are automatically created when any three or more intersecting lines or edges are in the same plane (an infinite flat 2D space), or coplanar.
The edges that bound a face remain when you delete a face. However, a face is erased automatically when you delete one of its bounding edges. SketchUp will create new edges and faces using the Auto-fold feature if you alter one of the edges of a face so that it is no longer co-planar with that face.
Draw faces with the Line Tool, Arc Tool, Freehand Tool, Rectangle Tool, Circle Tool, or Polygon tool. The following image was created simply by joining arcs and lines to form the edges and subsequent faces.