Once Renaissance cryptographers agreed that a plain shifted alphabet was too predictable, the next idea was natural: scramble the alphabet systematically, starting from a word both sides could remember. Keyed alphabets — keyword first, remainder after — appear in the sixteenth- century cipher manuals, among them Giambattista della Porta's De Furtivis Literarum Notis (1563), and stayed a standard ingredient of pen-and-paper cryptography for centuries.
The attack that dooms every fixed substitution was old news by then. Al-Kindi, a ninth-century Baghdad polymath, described breaking monoalphabetic ciphers by letter frequencies in a treatise on deciphering encrypted messages — keyword alphabets included, though Europe would not read him for centuries. Every keyed alphabet since lives under the same verdict: the mapping is fixed, so the counts betray it.
The most famous English lesson in substitution weakness is the correspondence that doomed Mary, Queen of Scots. Her letters with Anthony Babington used a fixed cipher alphabet (a nomenclator with a substitution alphabet at its core); Walsingham's cryptanalyst Thomas Phelippes read them by frequency analysis, and a forged postscript asking for the conspirators' names drew the final signatures. Mary was executed on the evidence. The plot's lesson — a fixed alphabet, however cleverly keyed, cannot survive a patient statistician — pushed Elizabethan cryptography toward codebooks with thousands of entries.
Keyword alphabets never vanished: they moved to puzzle books, war-time field training, and classrooms. As a key, they fail; as a memory aid, they are unmatched — a child can reconstruct the whole alphabet from one word, which makes the keyword cipher the natural first substitution with a real key to keep secret.