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.