Archtop windows that break... Revit has issues with circles. Sometimes it is very slick in how it maintains fillets, arcs connections and tangential references, other times it flips out at some of the simplest things, and these issues are sometimes random. The basic language, spring point, rise, radius, intrados, top, creast are for simple arches. This in geometric language is represented in Dr. Math's pages and on this reference to Chord Math from Open Math website. So we need to draw an arc- then make it flex so the width varies, but the rise is the same. Simple enough right? Until this happens... same family - at random points flips the arc: I cannot even begin to explain why this happens.Guessing a glitch in Revit. I have a workaround- or more regular results anyways- by teaching Revit Arc's and arches fundamental language: ARCH_r = (Width ^ 2 + 4 * H ^ 2) / (8 * H) Where: r is the radius of the arc Width is the width of the Chord