what esion has said is basically righth.

Imagine your 4 verts of a poly lie on a plane in space. If one or more of those points doesn't lie on the plan the polygon is said to be non-planar. This can cause you rendering problems as it screws how maya interprets this polygon. The simplest (but not necessarily th best) way to fix it is to triangulate the polygon (A triangle is ALWAYS planar)

next bit is taken from : https://caad.arch.ethz.ch/info/maya/m...yIntro.fm.html
# You can ensure planarity by setting the Keep New Faces Planar option on in the Tool Options menu, or by toggling Ensure Planarity on in the Tool Settings window for the Create Polygon Tool and the Append to Polygon Tool. See "Creating new polygons" on page 26, "Appending to polygons" on page 33, and "Keeping new faces planar" on page 140 for details.
# Non-planar quads can be fixed using the Split Polygon Tool (see Chapter 14, "Splitting and Subdividing Polygons" for details).


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