A digraph cipher encrypts by substituting each digraph (i.e. pair of letters) in the message with a different digraph or symbol. In the digraph cipher shown here, each plaintext digraph is substituted with a digraph from the square. For example, 'as' is encrypted by finding the intersection of the column headed by 'a' with the row headed by 's', which gives us NO. So, the plaintext digraph 'as' is substituted with the ciphertext digraph NO. This digraph cipher much harder to break than a single letter cipher, because the codebreaker has to identify the true value of 676 digraphs, as opposed to struggling with just 26 letter substitutions.