A table of the frequencies of letters in a given language is quite handy for breaking simple substitution ciphers or for devising static Huffman coding tables. According to http://library.thinkquest.org/28005/flashed/thelab/cryptograms/frequency.shtml, the frequencies for the English language are approximately

1. 7.3%
2. 0.9%
3. 3.0%
4. 4.4%
5. 13.0%
6. 2.8%
7. 1.6%
8. 3.5%
9. 7.4%
10. 0.2%
11. 0.3%
12. 3.5%
13. 2.5%
14. 7.8%
15. 7.4%
16. 2.7%
17. 0.3%
18. 7.7%
19. 6.3%
20. 9.3%
21. 2.7%
22. 1.3%
23. 1.6%
24. 0.5%
25. 1.9%
26. 0.1%

This source gives "etnrioasdhlcfpumygwvbxkqjz" in descending order of frequency. "E.T.", "Sanrio", and "Internet" can be found within the most common letters.

The book Codes, Ciphers and Secret Writing by George Beal gives the following information about frequency tables used for cracking simple substitution ciphers:

Letters in order of frequency (highest to lowest):
E T A O N R I S H D L F C M U G Y P W B V K X J Q Z

The letters can be grouped further by their frequencies:
Very Common: E
Common: T
Next most common: A O N R I S
Less Common: H
Less Common Still: D L F C M U
Rare: V K X J Q Z

Other frequency tables have been worked out for pairs of letters:
TH HE AN RE ER IN ON AT ND ST ES EN OF TE ED OR TI HI AS TO AR OU IS IT LE NT RI SE HA AL DE EA NE RO OM IO WE VE TA TR CO ME NG MA CE RA IC NS UT US BE UN CH WA SI LA AD LI RT CA NC SO NC SO LL UR EL RS EM AC IM PR TT OT WI EC

The most common words in English are:
THE OF AND TO IN A IS THAT FOR IT BY ARE BE WAS AS HE WITH HIS

Anyways all of these tables can be used to decipher substitution ciphers. If a certain character appears a lot you might start off by assuming it's an 'e' and work forwards from there using common pairs and words. This is called frequency analysis. Doesn't always work but it's a good place to start right? Of course when used in conjunction with a transposition cipher, that could mess you up for a long time.

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