This 18mm male to 18mm female diffused downstem is a wonderful choice for aهتfemale-jointed water pipe or dab rig. For those who appreciate thorough filtration with their glass piece, this 6-slitted downstem is exactly the kind of accessory you're looking for. Regarding the varying lengths of the 18mm to 18mm Diffused Downstem, the length is measured from above the ground joint to the bottom of the stem. Check out the measurement guide, within the product photos, for a great visualization.