An operator for opening and closing pivotal windows comprises at least two pull and pressure transferring, flexible linkage mechanisms (3,4) connected at one end to a fixture (5,6) on a frame member and at the opposite end in engagement with a driving device (12-17) mounted on the opposite main frame member (2). The driving device is common to the linkage mechanisms and is together with the parts of the linkage mechanisms engaging it mounted in a common housing (7) for receiving the entire length of the linkage mechanisms (3,4) and provided with guides (8,9) and lead-in openings (8a,9a) for the linkage mechanisms (3,4). A carrier element (10,11) in connection with the guides is via transmission elements (12-16) in the housing 87,28; 54) connected with a common driving unit (17).