Tabula recta

Tabula recta is a cryptographic term invented by Johannes Trithemius[?] in 1518. The tabula recta was a square table of alphabets, each one made by shifting the previous one to the left, like this:

  |ABCDEFGHIJKLMNOPQRSTUVWXYZ
 -+-------------------------- 
 A|ABCDEFGHIJKLMNOPQRSTUVWXYZ 
 B|BCDEFGHIJKLMNOPQRSTUVWXYZA 
 C|CDEFGHIJKLMNOPQRSTUVWXYZAB 
 D|DEFGHIJKLMNOPQRSTUVWXYZABC 
 E|EFGHIJKLMNOPQRSTUVWXYZABCD 
 F|FGHIJKLMNOPQRSTUVWXYZABCDE 
 G|GHIJKLMNOPQRSTUVWXYZABCDEF 
 H|HIJKLMNOPQRSTUVWXYZABCDEFG 
 I|IJKLMNOPQRSTUVWXYZABCDEFGH 
 J|JKLMNOPQRSTUVWXYZABCDEFGHI 
 K|KLMNOPQRSTUVWXYZABCDEFGHIJ 
 L|LMNOPQRSTUVWXYZABCDEFGHIJK 
 M|MNOPQRSTUVWXYZABCDEFGHIJKL 
 N|NOPQRSTUVWXYZABCDEFGHIJKLM 
 O|OPQRSTUVWXYZABCDEFGHIJKLMN 
 P|PQRSTUVWXYZABCDEFGHIJKLMNO 
 Q|QRSTUVWXYZABCDEFGHIJKLMNOP 
 R|RSTUVWXYZABCDEFGHIJKLMNOPQ 
 S|STUVWXYZABCDEFGHIJKLMNOPQR 
 T|TUVWXYZABCDEFGHIJKLMNOPQRS 
 U|UVWXYZABCDEFGHIJKLMNOPQRST 
 V|VWXYZABCDEFGHIJKLMNOPQRSTU 
 W|WXYZABCDEFGHIJKLMNOPQRSTUV 
 X|XYZABCDEFGHIJKLMNOPQRSTUVW 
 Y|YZABCDEFGHIJKLMNOPQRSTUVWX 
 Z|ZABCDEFGHIJKLMNOPQRSTUVWXY 

Trithemius used the tabula recta to define a polyalphabetic cipher which was equivalent to Leon Battista Alberti's cipher disk. The tabula recta is often referred to in discussing pre-computer ciphers, including the Vigenère Cipher and Vigenère's less well-known (but much stronger) autokey cipher. All polyalphabetic ciphers based on Caesar ciphers can be described in terms of the tabula recta.