A polygon with 10 sides
In a deck of cards the highest number is a 10 (Dec has the same word root as decade which is 10 years).
All 10 sides of a regular decagon are equal in length.
The internal angle between the sides of a regular decagon is 144°.
An irregular decagon such as the one above may be convex as in this case.
The decagon above is an irregular concave decagon.
This star shape is an irregular decagon as the internal angles between the sides are not all equal.