An interferometer consisting of a plane-parallel refractive plate rotatable with respect to a fixed flat mirror measures angle by the passage of fringes. It is an amplitude division two- beam interferometer like the one discovered by David Brewster in 1815. The folded Brewster interferometer is insensitive to errors of manufacture and assembly. It responds to movements in just one degree of freedom while being insensitive to movements in the other five. The dependence of retardation upon input angle is analyzed as a function of the design parameters. There is a refractive index which minimizes the moment of inertia of the moving element. Two ways to deploy sources and photosensors for angle measurement are discussed. A prototype folded Brewster interferometer built for a galvanometric scanner had a refractive plate measuring 3mm thick X 6 mm wide X 19 mm long. At (lambda) equals 633 nm one fringe corresponded to 128 (mu) radians, and an angular range of +/- .15 radian could be covered.