wall 14 of the casing 13 when the shaft 1 is rotated in one direction the ratchet wheel 20 entrains the associated pawl 18 and causes the casing 13 to rotate the sleeve 12 with the crossheads 9 in one direction i e the slats 25 are tilted in a first direction if the shaft 1 is rotated in the opposite direction the ratchet wheel 21 causes the associated pawl 19 to rotate in the same direction whereby the casing 13 causes the sleeve 12 to turn the crossheads 9 in the opposite direction i e the slats 25 are tilted in a second direction the parts 1 2 3 and 7 constitute a means for moving the slats 25 up and down between upper and lower end positions in which the venetian blind is respectively fully contracted and fully extended the parts 9 10 11 and 12 constitute the aforementioned turning mechanism which can change the angular positions of the slats 25 between first and second end positions respectively shown in fig5 and 7 and through a practically infinite number of intermediate positions including that which is shown in fig6 a stop 22 which is fixedly mounted at a level below the sprocket wheels 20 21 has two suitably inclined disengaging surfaces 23 and 24 when the shaft 1 is rotated in a counterclockwise direction as viewed in fig5 to raise the slats 25 i e to reduce the length.