Triangles are an engineers fraternity.
Some of each. You have to have a technical major, math, science, engineering, but otherwise it operates much like any other fraternity. Perhaps with a bit less of the hazing and hard drinking that characterized more "Greek" Houses. A friend of mine was a member, but a non residential one, since his folks lived in the same town as the University, as did mine. He is a friend that goes back to Junior High. We're both Grandpas now. )