An eleven sided shape is known as Hendecagon or undecagon. It is a polygon that has 11 sides and internal angles of 147.27 degrees.
An important aspect of a hendecagon is that it can not be constructed with a compass and straightedge. The area of a hendecagon is as follows:
A = 11/4 a2 cot (pie/11) - 9.36564 a2
A common example of a hendecagon is the Canadian dollar coin called loonie. The Indian two rupee coin is also formed from a hendecagon.