 As the Mach number is a dimensionless quantity rather than a unit of measure, the number comes "after" the unit; the second Mach number is "Mach2" instead of "2Mach" (or Machs).This is somewhat reminiscent of the early modern ocean sounding unit "mark" (a synonym for fathom), which was also unit-first, and may have influenced the use of the term Mach.In the decade preceding faster-than-sound human flight, aeronautical engineers referred to the speed of sound as "Mach's number", never "Mach 1".