The nozzles and brushes are sized to the diameter of the vacuum cleaner tube. In Zelmer * vacuum cleaners the brushes are mounted inside the tube, the outer diameter of the brush is 31.5 mm, the inner tube diameter is 32 mm. In other vacuum cleaners the brush is mounted on the pipe, the inside diameter of the brush should be slightly larger than the outer diameter of the pipe.
You can also choose a universal brush that will fit any vacuum cleaner by adjusting the clamping diameter (30-38 mm).