Q. I noticed that the hornbook pictured on page 12 of the
May 2008 Family Tree Magazine has a 27-letter alphabet, with a unknown letter between
r and
s. What’s the story?
A. The 18th-century English hornbook shown in our May 2008 History Matters column (here’s the hornbook—
it's from the Library of Congress Rare Book and Special Collections division) features a character called the long
s.

The long
s, which looks like a lower-case
f, was common in 18th-century England and Colonial New England. It was often used as an
s at the beginning or in the middle of a word (as in
fentiment), or as one or both letters of a double
s (
congrefs).
The long
s was not generally used as the final letter of a word—for that, people used the familiar short, or terminal,
s.
The long
s fell out of use around 1800 in England and 1820 in the United States.
For more on the long
s, see
Wikipedia's well-illustrated article and the book
Researching Your Colonial New England Ancestors By Patricia Law Hatcher (Ancestry, $16.95).
The book is available for a limited preview in Google; I've added it to
Family Tree Magazine’s Google Library for your linking convenience.