Basically, 3 Robertson squares are superimposed in a successive thirty° rotation. The design is comparable to that from the double-sq., in both of those cases The concept being that it resembles a square (Robertson) https://aoifemzwq253588.tnpwiki.com/6136408/how_much_you_need_to_expect_you_ll_pay_for_a_good_types_of_screw_heads