This cipher is one of the easiest substitution ciphers to write and to break. Because it is easy to break, it is not used very often.
You write 1 through 26 and write the alphabet next to the numbers – A=1, B=2, Y=25, Z=26. To make the code more difficult, you could write the alphabet backwards. Then 1 would equal Z, 2=Y, 25=B and 26=A.