text: how to trisect an angle? like an straightangle can easily be trisected .how to trisect an angle other than a straight angle? you can't trisect with an unmarked straightedge and compass. it *can* be done with a marked straightedge and compass. the key is being able to *slide* the marked straightedge. here's how it goes:  step i: draw a circle with radius equal to the length of the mark on the straightedge and with center the corner, o, of the angle you want to trisect. let the sides of the angle meet the circle at a and b.  step ii: extend the line oa through the other side of the circle.  step iii: place the straightedge so that one mark is on the extended line oa, one mark is on the circle and so that the edge goes through the point b. this is the step that is impossible without the marks on the straightedge. let the intersection with the extended line oa be at the point c.  then the angle acb will trisect the angle aob.