The Relationship between SketchUp Model’s Edges and Faces
If you know that models are made from edges and faces, you’re most of the way to understanding how SketchUp works. Here’s some information that should fill in the gaps:
1Every time SketchUp can make a face, it will.
There’s no such thing as a Face tool in this software; SketchUp just automatically makes a face every time you finish drawing a closed shape out of three or more coplanar edges. As soon as you connect the last edge that you draw to the first one to close the loop, SketchUp creates a face.
2You can’t stop SketchUp from creating faces, but you can erase them if you want.
If SketchUp creates a face you don’t want, just right-click the face and choose Erase from the context menu. That face is deleted, but the edges that defined it remain.
3If you delete an edge that defines a face, that face will be deleted too.
When you erase one of the edges in the cube (with the Eraser tool, in this case), both the faces that were defined by that edge disappear. This happens because it’s impossible to have a face without also having all its edges.
4Retracing an edge re-creates a missing face.
If you already have a closed loop of coplanar edges but no face (because you erased it, perhaps), you can redraw one of the edges to make a new face. Just use the Line tool to trace over one of the edge segments, and a face reappears.
5Drawing an edge all the way across a face splits the face in two.
When you draw an edge (like with the Line tool) from one side of a face to another, you cut that face in two. The same thing happens when you draw a closed loop of edges (like a rectangle) on a face — you end up with two faces, one inside the other.
In the figure, a face is split in two with the Line tool and then one of them is extruded out a little bit with the Push/Pull tool.
6Drawing an edge that crosses another edge automatically splits both edges where they touch.
In this way, you can split simple edges you draw with the Line tool, as well as edges created when you draw shapes like rectangles and circles. Most of the time, this auto-slicing is desirable, but if it’s not, you can always use groups and components to keep things separate.