The common incircle of the medial triangle (left figure) and the congruent triangle , where are the midpoints of the line segment joining the Nagel point Na with the vertices of the original triangle (right figure).
The Spieker circle has circle function
which does not correspond to any named center. The center of the Spieker circle is called the Spieker center Sp, and the circle has radius
where is the inradius and is the semiperimeter of the reference triangle.
The Spieker circle passes through Kimberling centers for , 3036, 3037, 3038, 3039, 3040, 3041, and 3042.
The common area of Wolfram Research's front-end programming group contains a triangular table illustrating the construction of the Spieker circle. This table was built by Theodore Gray, director of user interfaces at Wolfram Research, using walnut and inlays of maple, the latter of which was obtained from a tree formerly standing in the yard of front end developer Chris Carlson. The triangular table has sides lengths (3, 4, 5), a Pythagorean triple. The larger inlaid circle is the incircle of , with the incenter representing the point of concurrence of the triangle's angle bisectors. The smaller inlaid circle is the Spieker circle, which can be seen to correspond to the incircle of the medial triangle . The triangle's cleavers are also shown, and concur in the Spieker center (which is therefore also the cleavance center). E. Pegg Jr. has posted a photo history of the construction of this table.