3D printers build objects by solidifying (SLA, SLS, SHS,DMLS, EBM, DLP) or printing (3DP, MJM, FDM, FFF, PJP, MJS) one layer at a time. This requires a series of closed 2D contours(horizontal layers) that are filled in with solidified material as the layers are fused together. A natural file format for such a machine would be a series of closed polygons (layers or slices) corresponding to different Z-values. However, since it is possible to vary the layer thicknesses for a faster though less precise build, it was easier to define the model to be built as a closed polyhedron that can be sliced at the necessary horizontal levels. An incorrect facet normal can affect the way a file is sliced and filled. A slice at a different Z-value can be chosen to miss a bad facet or the file must be returned to CAD program to make corrections and then regenerate the STL file.