It's well known that there is no possibility to trisect and angle with a compass and a ruler. But there is such a procedure when the ruler has 2 marks on it. See the snippet below. It's not very intuitive but probably correct since the source is trustful: RJ Lipton: P vs. NP and the Godel's lost letter. I'm curious about a reference which gives this extended tools trisection. Or even proof at this place.
