An integral part of the Zone System as taught by Ansel Adams and Minor White was the use of a Weston light meter. "The Weston exposure meter, because of its convenient dial, is the pivotal instrument used in the zone system." So states Minor White in his Zone System Manual.
As for the zone system itself, the key concept was previsualization, being able to see, in your mind’s eye, what your black and white print was going to look like before you tripped the shutter:
. . . the zone system—one must sleep it, eat it, drink it, see it, mold it with every impression until previsualization is a constant state of existence whether one is making photographs with a camera, with the eyes alone—or with remembered images, eyes closed.
Again, Minor White.
My old Weston light meter, which I still have, is shown above. It still is somewhat sensitive to light, though not enough for practical use.
Alas, even though I was using a Weston meter, my photos never seemed to measure up to either Mr. Adams' or Mr. White's.
Photo: Weston Meter—Marin County, 2009