I just measured mine. The Clearview Super 60 and Marti Michell's Equilateral Triangle Ruler both measured exactly right (when compared to each other and to my Omnigrid ruler). However, the EZ 60 degree diamond ruler from Darlene Zimmerman, an orange ruler which has two 60 degree ends (and is sold at JoAnn's), was off on one end. I've never actually used that ruler (lucky for me) but I have used the other two quite a bit, and never had a problem with them.