The book has 2 main sections - descendants of immigrant Henry Dow, and descendants of immigrant Thomas Dow.Henry has been assigned the letter "a" and Thomas (of Newbury) the letter "b."Children are assigned an additional letter based on birth order.The first child of Henry is aa, 2nd is ab, third is ac - and so on. This means Thomas ah is the eighth child of immigrant ancestor Henry Dow.Henry is described in detail on pages 27 through 34.Note that on page 24 he is referred to as xaaf, but though the remainder of the book he is referred to as a.
This is because the book starts with an ancestor of Henry called John of Yarmouth (p. 24). He had 3 sons: Thomas, Christopher, and Katherine. Son Thomas was the next in the line, and since he is the first child of John, he is listed as xa (p. 25). His first son was also called Henry and is referred to as xaa (p.26). The 5th child of Henry xaa was bapt. in 1605 and was the Henry Dow who came to America and settled first in Watertown MA in 1637. Since he is the 5th child of Henry xaa, he is referred to as Henry xaaf in the introductory part of the book (p. 27).From then on he is assigned just the letter a, and all his progeny have a letter key starting with the letter a.