Most secret languages are not true languages at all, but cyphers. A cypher keeps the word order and structure of the original language, but either replaces the words with new ones, or changes them by some transformational rule. Such a language may not fool serious code-breakers, but it will stop people not in on the secret from understanding what you are saying.
A Simple Secret Code
Write out all the letters of the alphabet on a sheet of paper. If it takes you multiple lines, be sure to double-space.
Write the letters of your substitution code underneath. For example, write W under A, L under B, etc.
Make a copy of the substitution code for any friends you want to be able to read your secret messages.
When you want to write a secret message, substitute the code letters for the ones you would normally use. For example, when you want to write the word "the," write the letters written under "t," "h" and "e."
To decode a secret message, substitute the real letters for the encoded ones. Look up each letter in the message in the bottom row of your decoder sheet, and write the correct substitute letter on the message.
Secret Spoken Language
Add a sound on to the beginning of every word. For example, a cypher called Snenglish could start every word with "sn." So, "Call me Ishamel." becomes "Snall snee Snishmael." Although this sort of cypher is easy to understand, when spoken quickly enough it can confuse people who aren't expecting it.
Come up with more complicated ways to change words. For example, in "Op Talk," you spell out the words, adding the syllable "op" after every consonant sound and before every word that starts with a vowel. For example, the phrase "This is getting tiring" would be "Thop I sop op i sop gop e top top i nop gop top i rop i nop gop."
Change the order you say the words in. For example, in Pig Latin you move any consonant at the beginning of the word to the end of the word and say "ay" after it. So the phrase "You probably know this already" becomes "Ou-yay robably-pay ow-nay is-thay already-ay."
Come up with substitute versions of everyday words. People do this all the time and it is called slang. When you do it intentionally and only let a few people in on it, it can be a good way to keep your communication secret.
A simple substitution code, where each letter is always changed to a particular alternate letter, is very easy to crack. To make your secret code more robust, check out the link below.
Check out Cockney Rhyming Slang for some good ideas. In Cockney slang, rhymes are used as substitutes for everyday objects. For example, you might say "uncle Ned" for "bed."
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