From the Riddles of Ancient Egypt to Cryptography in the Renaissance — 3500 Years in the Making - Codes: The Guide to Secrecy from Ancient to Modern Times - page 48

same plaintext symbol. For instance, with the Caesar cipher in Table 1.2 (page
11), the letter D is always the ciphertext for the plaintext letter a ,so D is a
homophone in the monoalphabetic cipher known as the Caesar cipher. Here
“monoalphabetic” means that there is only one cipher alphabet , which means
the set of ciphertext equivalents used to transform the plaintext. The row of
ciphertext equivalents below the plaintext in Table 1.2, for instance, is the cipher
alphabet for the Caesar cipher. A polyphone is a ciphertext symbol that always
represents the same set of plaintext symbols, typically a set consisting of at most
3 plaintext symbols. With homophones or polyphones, there is no option for
change since the relationship between plaintext and ciphertext is fixed. However,
a cipher is called polyalphabetic if it has more than one cipher alphabet. In this
type of cipher, the relationship between the ciphertext substitution for plaintext
symbols is variable. Thus, since each cipher alphabet (usually) employs the same
symbols, a given symbol may represent several plaintexts.
Alberti conceived of a disk with plaintext letters and numbers on the outer
ring and ciphertext symbols on an inner movable circle. Alberti divided his ring
and corresponding circle into 24 equal segments, called cells , each containing a
symbol.
A representation of Alberti's disk is pictured in Figure 1.22. We have altered
his original presentation since he had ciphertext in lower case and plaintext in
upper case, the reverse of what we have as a convention.
Figure 1.22: Alberti disk.