A quadruple of points on a straight line with cross ratio equal to $-1$. If $(ABCD)$ is a harmonic quadruple of points, one says that the pair $AB$ harmonically divides the pair $CD$, or that the points $A$ and $B$ are harmonically conjugate with respect to the points $C$ and $D$; the pairs $AB$ and $CD$ are called harmonically conjugate.
A harmonic quadruple can be defined without recourse to metric concepts. Let $PQRS$ be a quadrangle (see Fig.), let $A$ and $B$ be the intersection points of the opposite sides, and let $C$ and $D$ be the intersection points of the diagonals $SQ$ and $PR$ of $PQRS$ with the straight line $AB$. Then the quadruple of points $(ABCD)$ is a harmonic quadruple. A quadruple of straight lines (or planes) passing through a single point (a single straight line) is called a harmonic quadruple of straight lines (planes) if a straight line intersects it in a harmonic quadruple of points.
When the straight line is a complex one, but viewed as a Euclidean plane, one says harmonic quadrilateral, see [a1].
|[a1]||M. Berger, "Geometry" , I , Springer (1987) pp. 270|
|[a2]||H.S.M. Coxeter, "Projective geometry" , Blaisdell (1964)|
|[a3]||H.S.M. Coxeter, "The real projective plane" , McGraw-Hill (1949)|
Harmonic quadruple. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Harmonic_quadruple&oldid=31972