Zone Ruler
Pentaxuser,
A little late getting back to you, but here is the answer to your question.
A Zone Ruler is simply a "ruler" which contains actual squares of photographic paper in Zones 1-9 or whatever, in one-stop steps from black to white. They need to be made using "proper proofing times" in order to be helpful.
First film speed is determined, then development time determined to place Zone VIII where you want it (in Zone VIII for N, in Zone IX for N+1, etc.). Then, film is exposed on Zones I through IX for N, (fewer or more Zones for N+ and N- developments respectively). This film is then printed at "proper proofing time" (usually contact printed) onto photo paper and, voilà, you have ten shades from black to white that shows exactly how that film/development/paper combination respond. Squares of paper for each Zone can be mounted on a long piece of board to make the Zone Ruler. Zone Rulers are handy for people just learning the Zone System to help them visualize just what, say, Zone IV looks like.
Zone Rulers are also useful, since the middle "Zones" on the paper rarely ever correspond to the ideal, and vary quiet a bit depending on the combination. Knowing that a particular combination separates a lot in the mid-tones, but has less shadow separation (or vice-versa) can be helpful when visualizing and metering.
Of course, I use sheet film and a spot meter to determine the development for each sheet, but making Zone Rulers can also be a helpful exercise with roll film to get an idea of exactly how your particular combination renders tones across the scale from black to white.
Hope this helps,
Doremus Scudder
www.DoremusScudder.com