Proportion tutorial: dodecagon II
A superior construction of the dodecagon: fewer actions than previously.

Start with a circle. Make perpendicular lines. Get the 4 intersecting points; use them as centers.

Get the intersections of the original circle with the new ones: dodecagon.

The source code of this page has more information.
var p3 = theP.addFirstIntersection(c1, c2, p1);