In cryptography, a means of decrypting ciphers first developed by Arab mathematicians in the 9th century. As letters in a text have certain frequencies (for example E and T are the two most common letters in English), their encrypted forms will appear as commonly in a suitably long piece of ciphertext, which will restrict enormously the possible ciphers to consider. Modern cryptographic methods, such as RSA, are still essentially ciphers but work on an ‘alphabet’ of size 2⁴⁰⁹⁶, rather than 26, and so are not susceptible to frequency analysis.