Blender.org

Hi. I'm new to Blender, and to CAD in general, and need help getting started. I have very little experience with CAD and scripting-I'm not even any good with Google Sketchup. I need help learning to use Blender so that I can make the most of it.

At the moment, I have a problem with editing shapes. I started with a cube, and tried dragging 2 of the corners out to change the shape. However, despite doing the same thing to both corners (albeit in different directions) they behaved differently. One seemed to add in an extra (unselectable) edge, whilst the other didn't, and I couldn't find any way of fixing it.

Can anyone tell me what's going on?

Thanks.

In edit mode are you selecting vertices or edges or faces?

Vertices.

In 3d graphic averything is made out of triangles.
Quad is made out of 2 triangles.
If all vertices of quad are coplanar you don't see edge between those two trangles. If you offset one vertex in a way that the 4 vertices that make quad are now incoplanar and if you look at right angle you will see edge - two "hidden" triangles.

p.s. i tryed what you did myself and you probably moved two vertices in a way that cube triangles are intersecting.