An adjustable conveyor guide rail unit adapted for use in a conveyor guide to define the side of a channel through which an article may be conveyed, the guide rail unit comprising: a fixed-position support; a carriage operable to carry a guide member; and a position-setting mechanism comprising a shaft extending in a direction that is not parallel with a width of the channel and a rotatable part mounted on the shaft; wherein the carriage is coupled moveably to the fixed-position support via the position-setting mechanism, such that the carriage and hence guide member are moveable relative to the fixed-position support to allow the guide member to be placed in a plurality of discrete positions to allow the width of the channel to be varied; the position-setting mechanism is arranged to set the carriage and hence the guide member into a selected one of the plurality of discrete positions using a complementary pin and hole arrangement where a pin is received in a hole, the pin and hole arrangement defining a plurality of pin/hole combinations to provide the plurality of discrete positions of the guide member, characterized in that the pin and hole arrangement is such that the plurality of pin/hole combinations are provided by a plurality of pins arranged on a variable radius relative to the shaft of the rotatable part and/or a plurality of holes arranged on a variable radius relative to the shaft of the rotatable part thereby ensuring that rotation of the rotatable part about the shaft allows different pin/hole combinations to be brought into alignment, and wherein either the pin and hole arrangement is provided by the rotatable part and the carriage, and the position-setting mechanism is mounted to the fixed-position support or the pin and hole arrangement is provided by the rotatable part and the fixed-position support, and the position-setting mechanism is mounted to the carriage; and wherein the rotatable part is movably mounted on the shaft such that the rotatable part may be moved along the shaft from a closed position in which a pin is received within a hole thereby setting a position of the guide member, to an open position in which there is no engagement between a pin and a hole thereby allowing the carriage to move relative to the fixed-position support. a fixed-position support; a carriage operable to carry a guide member; and a position-setting mechanism comprising a shaft extending in a direction that is not parallel with a width of the channel and a rotatable part mounted on the shaft; wherein the carriage is coupled moveably to the fixed-position support via the position-setting mechanism, such that the carriage and hence guide member are moveable relative to the fixed-position support to allow the guide member to be placed in a plurality of discrete positions to allow the width of the channel to be varied; the position-setting mechanism is arranged to set the carriage and hence the guide member into a selected one of the plurality of discrete positions using a complementary pin and hole arrangement where a pin is received in a hole, the pin and hole arrangement defining a plurality of pin/hole combinations to provide the plurality of discrete positions of the guide member, characterized in that the pin and hole arrangement is such that the plurality of pin/hole combinations are provided by a plurality of pins arranged on a variable radius relative to the shaft of the rotatable part and/or a plurality of holes arranged on a variable radius relative to the shaft of the rotatable part thereby ensuring that rotation of the rotatable part about the shaft allows different pin/hole combinations to be brought into alignment, the pin and hole arrangement defining a plurality of pin/hole combinations to provide the plurality of discrete positions of the guide member, characterized in that the pin and hole arrangement is such that the plurality of pin/hole combinations are provided by a plurality of pins arranged on a variable radius relative to the shaft of the rotatable part and/or a plurality of holes arranged on a variable radius relative to the shaft of the rotatable part thereby ensuring that rotation of the rotatable part about the shaft allows different pin/hole combinations to be brought into alignment, and wherein either the pin and hole arrangement is provided by the rotatable part and the carriage, and the position-setting mechanism is mounted to the fixed-position support the pin and hole arrangement is provided by the rotatable part and the carriage, and the position-setting mechanism is mounted to the fixed-position support or the pin and hole arrangement is provided by the rotatable part and the fixed-position support, and the position-setting mechanism is mounted to the carriage; the pin and hole arrangement is provided by the rotatable part and the fixed-position support, and the position-setting mechanism is mounted to the carriage;