Core illusions. (a) A Müller-Lyer illusion, in its most popular form. The two gray segments have equal legths. (b) A variant of the Müller-Lyer illusion, proposed by Müller-Lyer himself, showing that contrary to popular belief, the fins are not necessary for the illusion. (c) A variation on the theme of ingoing and outgoing fins, proposed by Judd (). The dot is bisecting exactly the shaft, but appears displaced toward the ingoing fins. (d) A variation by Pegrassi () on the previous Judd illusion. (e) Sander's parallelogram (). The diagonal on the left side seems longer than the diagonal on the right. It seems that the illusion can already be demonstrated with two triangles (Figure 5h). (f) Delboeuf illusion with circles. In the central pair of circles, the internal small circle is equal to the small external circle, but appears larger, and the surrounding large circle is equal to the external large circle, but seems smaller. (g) Poggendorff illusion. The two thick oblique segments are collinear, but appear to be misaligned. (h) Helmholtz' square illusion. The squares are equal, but are perceived as rectangles that are elongated in the direction that is orthogonal to the dividing segments. (i) Oppel-Kundt illusion: The subdivided half of the figure appears to be longer than the non-subdivided half. (j) Ebbinghaus or Titchener's illusion (Ebbinghaus, ). The two central circles are equal, but the one surrounded by small circles seems to be larger than the other. The illusory effect does not require circular geometry. It already works with triples of aligned elements. (k) A variation on this theme (from Ninio, ), designed as a hybrid between (j) and (n) or Figure . (l) Variation on the bisection illlusion. The small isolated segments are equal in length to the segments that are associated in crosses, but appear to be longer (from Ninio, ). (m) Zöllner illusion. The two stacks are parallel, but seem to diverge at their tops (Zöllner, ). The “left-handed” and “right-handed” variants of the stacks, are labeled “L” and “R” according to the convention in Ninio and O'Regan (). (n) The so-called Ponzo illusion. The horizontal bars seem to increase in length, from bottom to top. (o) Hering illusion. The long line crossing the bundle is straight, but seems to be curved. (p) Variation on a theme by Ehrenstein () and Orbison (): The black square seems to be trapezoidal, due to its intersections with the circles.