Abstract: Apparatus for the individual control of the raising and lowering movement of warp yarns on a fabric weaving loom, in which the warp yarns are passed through guide eyes, these eyes being mounted on the piston rods of fluid pressure operated piston and cylinder jacks. There are a large number of jacks, these being grouped in a plurality of units which are removable and are juxtaposable with respect to number and position in accordance with the desired density of warp threads, the units being arranged in at least two tiers. This gives great flexibility of control.