Definition.—If tivo angles hare the same vertex and the same bisector, the sides of either angle are isogonal to each other with respect to the other angle.

Thus the isogonal of AP with respect to ∠ BAC is the image of AP in the bisector of ∠ BAC. It is indifferent whether the bisector of the interior ∠ BAC be taken, or the bisector of the angle adjacent to it; the isogonal of AP remains the same.