Downstems have three key measurements:

Length - Measured from top of ground joint to bottom of stem in inches

Diameter - Measured at top of ground joint in millimeters, this is the width of the downstem where it will enter the Water Pipe, shown as number before the /, 

e.g 18/14 = 18mm

Slide/Bowl - Measured at inner diameter of top opening in millimeters, this is where your slide goes, shown as number after the / 

e.g 18/14mm = 14mm slide, 24/18mm = 18mm slide