a symbol to change the direction not only to left and right, but also upward and downward is required. In our environment, we use ‘(a #)’ symbol to change the direction upward and downward. If this symbol is used before the symbols ‘m’, ‘o’ or ‘O’, the newly created face is rotated towards the previous face, making a dihedral angle. Both positive and negative number can be put in ‘a’ and the direction changes upward at an angle of ‘a’ if a positive number is used and changes downward if a negative number
is used.