| And two circles are tangent to a straight line. Common tangent of two circles in the same side, known as the grandfather tangent; two circles on both sides of the common tangent, the period known as the common tangent. Common tangent between the two cut points, known as the common tangent of the long distance. If the grandfather of two tangent circles have two (or within the common tangent), then the tangent of the long-grandfather (or the common tangent of length) are equal. |