11-sides. The regular polygons of 7, 9 and 11 are not constructable with ruler and compass. A more complete discussion of this will be found in the Hendecagon article which follows. Here I will just show two animations with also be discussed in that article. One is called a swirl and the other is the hendecastar.
Here is the swirl , As you can see other convex polygons can have their swirls as well.
Here is the hendecastar.
