Letter frequency varies by language, of course. It also varies according to topic. If you're speaking about jail and jokes told in jail, you'll use the letter "j" far more often than it is normally used. There are some differences between dialects, due to different word frequency and spellings. For example, the ending "ise" is more common in British English, while "ize" is used more in American English. As a result, the use of "z" is more common in American English.

Finally, everyone writes differently, a fact sometimes used to prove or disprove authorship, since the average sentence length, word length, and letter frequencies can be determined for each author. A forger can rarely forge an authors "style," as determined in part by these measurements. Interestingly, during World War Two, British code breakers used this kind of analysis to identify not just what a secret message said, but who was sending it.

Below are the letters of the English alphabet, arranged in order of prevalence (on average). Using a frequency table like this is the first step in code breaking.

e-t-a-o-i-n-s-h-r-d-l-c-u-m-w-f-g-y-p-b-v-k-j-x-q-z

The simplest secret codes simply replace a letter with another (w = a, x = b, d = c, ...). Periods and other punctuation may or may not be used. Simple code breaking then, begins with identifying the most common letter in a message and replacing each of these with an "e", since that is the most common and therefore most likely letter. If that doesn't seem to work, a "t" or "a" would be tried next.

There are other ways to speed up the process. As you look at the coded message below, for example, you can see a few words that are just one letter. The obvious candidate is "a", although it could be the letter "i". A three-letter word starting several sentences is likely to be "the", in which case you would know three more of the letters. Starting with these simple code breaking guidelines, see how quickly you can break the code and read the message below.

lbyeavb ha hpb lagyc at eacb sgbxzqwr. hpb zbm, xi mak vxm pxjb wahqebc, qi ha ihxgh lqhp hpb iqvdyb gkybi xsakh ybhhbg tgbfkbwem, xwc hgm wbl xddgaxepbi awym lpbw hpbib txqy. qw ahpbg lagci, mak dyxm hpb acci ha ihxgh, xwc hpbw xcukih makg ihgxhbrm xi wbbcbc.

hpqi qi x rgbxh sgxqw bnbgeqib. egbxhqwr makg alw, daiiqsym kwsgbxzxsyb eacb qi xyia x tkw lxm ha bnbgeqib makg sgxqw. hpbgb lqyy sbe vagb aw hpxh qw xw kdeavqwr qiikb at hpb sgxqwdalbg wbliwbhhbg. hpb iksiegqdhqaw tagv qi aw hpb pavb dxrb. lpm wah ra iqrw kd wal?

Of course no good code creator would make such a simple code, so don't get too excited if you figured this one out. Breaking a tough code requires a bit more knowledge than the simple rules laid out here. I'll have more on that, and how to create your own "unbreakable" codes, in a future article on code breaking.