Table 2.7 Symbols of crystallographic point groups. Crystal First position Second position Third position Point group system Element Direction Element Direction Element Direction Triclinic 1orI N/A None None 1,i Monoclinic 2,m or 2/m None None 2,m,2/m Orthorhombic 2 or m X 2 or m Y 2 orm Z 222,mm2,mmm Tetragonal 4,4or Z None or X None or Base 4,4,4/m,422, 4/m 2orm 2 or m diagonal 4mm,42m, 4/mmm Trigonal 3 or3 Z None or X None 3,3,32,3m,3m 2orm Hexagonal 6,6 or 6/m Z None or X None or Base 6,6,6/m,622, 2 orm 2 orm diagonal 6mm,62m, 6/mmm Cubic 2,m,4 X 3or 3 Body None or Face 23,m3,432,43m, or4 diagonal 2 or m diagonal m3m 2m表示其对称面与2次轴相垂直，其余类推 55 Z 2/m表示其对称面与2次轴相垂直，其余类推