The molecule is said to belong to the D3h POINT GROUP.
Let us now get a bit more general, and call the principal axis Cn, so that its order, n, can be any number.
If there is no horizontal plane of symmetry, but there are n vertical planes as well as nC2 axes, the point group is D nd
The D and the number mean the same as before but the small d stands for DIHEDRAL PLANES, because the n vertical planes lie between the nC2 axes.
Ethane in the staggered conformation belongs to a Dnd point group. Decide on the value of n from the following diagram (looking down the principal axis), and hence state the point group to which ethane belongs.