TheCodeBreakers
The vowels
a, o, i, and the consonants t, n, r, s, and h will also be of high frequency,
while /', k, q, x and z will be of low frequency. This constancy suggests
that a frequency count of the next text will show the same letters with
the same frequencies—everi if that text happens to be concealed by a
cipher system. The cryptanalyst can utilize the known frequencies to
dislodge the text from the cipher.
In other words, given a monoalphabet substitution whose plaintext he
knows to be in English, the cryptanalyst will count the number of letters
in the ciphertext. If he finds that, say, L is the most frequent, he knows
by the nature of the cipher that it must stand for the most frequent letter
in the hidden plaintext. Since that is English, he can expect that that
letter is e. Consequently, he can assume that L represents e. This is the
basic process of cryptanalysis.
annaabi.ee 
