 The petrie polygons of the regular 4-polytope define regular skew polygons.The Coxeter number for each coxeter group symmetry expresses how many sides a petrie polygon has.This is 5 sides for a 5-cell, 8 sides for a tesseract and 16-cell, 12 sides for a 24-cell, and 30 sides for a 120-cell and 600-cell.