Symmetry operation

The most basic operation is "E" - the "Identity" operation, Leaves the object unchanged. Other operations are: "Cn"-Rotation about an axis, The rotation axis is an element of symmetry, An axis is denoted Cn if a(360/n) degrees rotation around the axis is equal to the "E" operation(e.g An equilateral triangle, has a C3 rotation axis that runs through the middle, perpendicular to the 3 triangle's lines. the lines the runs through the center, from every intersection of 2 lines marks the positions of three C2 rotation axis .) "σ" denotes a vertical, horizontal or diagonal [Reflection (mathematics)] plane of symmetry "i" - [Inversion in a point] "Sn" - [Improper rotation] : rotation by 2pi/n followed by reflection perpendicular to rotation axis ( Thus S1= σ, S2= i)

source : Inorganic chemistry, Shriver and Atkins, 3rd edition, p.118