Classical Cipher used in Classical Cipher used in World War I

Playfair Cipher

Polygraphic substitution is a cipher in which a uniform substitution is performed on blocks of letters

Playfair Cipher encrypts pairs of letters (bigrams or digrams), instead of single letters. This makes Frequency Analysis much harder, since there are $26^2$ possible bigrams rather than the $26$ possible monograms.

→ a considerably larger cipher text is required in order to be useful. The playfair cipher