In substitution ciphers, the letters are changed, but not the order in which they appear.

Monoalphabetic means that one letter switchs with one other letter, and that's it. The most common is...

Using the cipher above, "Myrtle has big feet" would be "Nbigov szh yrt uvvg." Having words like "big" in the ciphertext (Nbigov) can confuse people trying to decipher the text.

Other common methods are switching letters for numbers (where "A" either becomes "1" or "26").

The more you mix up the encryption, the harder it will be to crack (but remember that the person reading it needs to be able to read it easily).

For example, the capital letters could be encrypted like this...

...where "A" (plaintext) translates to "N" (ciphertext), and "N" (plaintext) translates to "A" (ciphertext). For example, "ALPHABET" becomes "NYCUNORG."

But the lower case letters would be encrypted like this...

...where "a" (plaintext) translates to "z" (ciphertext), and "z" (plaintext) translates to "a" (ciphertext).

Using this method of encryting capital letters differently than lower case, we could translated "Joe is HOTT but Steve is NOT" like this: "Wlv rh UBGG yfg Fgvev rh ABG," rather than using just the capital letters system ("Wbr vf UBGG ohg Fgrir vf ABG") or just the lower case system ("Qlv rh SLGG yfg Hgvev rh MLG"), because one way cryptanalysis can decipher texts is by the pattern of words and the number of letters in each word. So, with the two separate encryptions, it makes it harder for unwanted eyes to read what you're trying to say.