Meaning of pitch line

pitch' line"

Pronunciation: [key]
— Mach. Mach.
  1. See
  2. an imaginary line within the profiles of the teeth of a rack, such that it moves against, and at the same rate as, the pitch circle of an engaging pinion.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.