a conveyance system has movable guide rails that can be positioned close to one another when small packages or articles are carried by the system or further apart from one another if the conveyed packages or articles are large . the spacing between the guide rails is controlled by a linkage arrangement that includes a center link mounted for conjoint rotation on a longitudinally disposed drive shaft . each opposite end of the center link is pivotally attached to an outer link and the outboard end of each outer link is pivotally mounted to a slideably mounted mounting block to which is secured a rail support for holding a guide rail . the drive shaft is rotated manually through a control knob or the rotation is performed by a reversible motor . alternatively , a bell crank rotates the drive shaft in a first direction to cause convergence of the guide rails and in a second , opposite direction to cause divergence of the guide rails . in a second embodiment , the conveyor system has a curvature formed in it . curved guide rails have guide rail extensions slideably mounted to them at their opposite ends to enable the guide rails to be effectively lengthened and shortened when the space between them is increased or decreased , respectively .