An operator for a casement-type window having a frame and a sash mounted by a slider adjacent an edge of the sash for rotational opening and closing movement about a slider pivot axis with linear movement of the slider. The operator has a first sector gear fixed to the sash with the gear center offset relative to said slider pivot axis and a second gear meshing with the sector gear and functioning as a drive gear with a first link functioning to hold said gears in meshed relation and a constraining link connectable between the sash and the window frame with the constraining link and said first link acting to guide the opening and closing movements of the sash.