vertices

At the vertices of a regular tetrahedron may be found such points.

The harmonic points are the vertices of the “harmonic triangle of the complete quadrangle.”

A four-side has six vertices, of which two and two are opposite, and three diagonals, which join opposite vertices.

If two pairs of adjacent vertices coincide, the hexagon becomes a quadrilateral, with tangents at two vertices.

Of a polar-triangle any two vertices are conjugate poles, any two sides conjugate lines.