Abstract: A window operator for a wide range of sizes of casement windows includes a housing in which a worm gear is mounted. An arm assembly has a gear segment that engages with the worm gear so that rotation of the worm gear causes the arm assembly and window sash to pivot outward or inward. The arm assembly includes three arms which are telescopically arranged. The inner arm and lead arm have outward facing races. The center arm has an inverted U-shape with downwardly extending legs of the U each have corresponding races. As the worm turns, the arm assembly rotates outward and the three arms slide with respect to one another.