1. 12.1 Tangents

10. When each side of a polygon is tangent to a circle, the polygon is said to be circumscribed about the circle, the circle is inscribed in the polygon.

13. Common tangent  a line that is tangent to each of 2 coplanar circles.

14. Common internal tangent a common tangent that intersects the segment joining the centers of the 2 circles.

15. Common external tangent  a common tangent that does not intersect the segment joining the centers.

16. A circle can also be tangent to another circle.
Tangent circles are coplanar circles that are tangent to the same line at the same point.