The top two lines in Chart II give the letters of the 23-letter alphabet
and the numerical position of each letter in that alphabet. Thus C is the
3rd letter in this alphabet, and Z [off my chart, but not yours] is the
23rd letter. Chart II tells you the number of leaves in partial
alphabets. Examples:
If the book is in quarto format and the collation is A-2D<4>, then
it will contain (as Chart I tells us) 92 leaves plus (as Chart II tells
us) 16 leaves, for a total of 108 leaves.
If the book is in octavo format and the collation is A-3E<8>, then
it will contain (as Chart I tells us) 368 leaves plus (as Chart II tells
us) 40 leaves, for a total of 408 leaves.
If the book is in 12mo format and the collation is A-4F<12>,
then it will contain (as Chart I tells us) 828 leaves plus (as Chart III
tells us) 72 leaves, for a total of 900 leaves.
No moving parts, and easily corrected if you get any of the sums wrong
when setting it up (as Horblit did, necessitating a MS correction).
I believe it was Michael Winship who first showed me his copies of these
simple charts, which he obtained from Jacob Blanck. Don McKenzie once
showed me similar ones of his own.
Somewhere along the line (from Richard Noble, I think) I picked up the
useful mnemonic: Every King Prefers Victory, helpful in remembering that
letters E, K, P, and V are the 5th, 10th, 15th, and 20th letters in the
23-letter alphabet. With the mnemonic, um, in mind, you can usually do
without the second chart, in a pinch.