In the figure above, chords QR and ST intersect. The theorem states that the product of QB and BR is equal to the product of SB and BT.
Every secant segment is divided into two segments by the circle it intersects. The internal segment is a chord, and the external segment is the segment with one endpoint at the intersection of the secant segment and the circle, and the other endpoint at the fixed point outside the circle. Given these conditions, a theorem states that when two secant segments share an endpoint not on the circle, the products of the lengths of each secant segment and its external segment are equal.
A similar theorem exists when a secant segment and a tangent segment share an endpoint not on the circle. This theorem states that the length of the tangent segment squared is equal to the product of the secant segment and its external segment.