Reading Mates is written using rhyme and a carefully graded sequence of letters. I chose the sequence of letters for several reasons.

The purpose of learning letters and their associated sounds is to read. So, it seems logical to choose letters that can be combined immediately to make words that can be read in sequence. The initial letters s, a, t, i and m, followed by n, o and p are used by many authors of decodable series because they can be combined into many words which beginning readers can decode.

These letters tend to be high frequency in that they occur often in English words. For instance, s is a letter whose sound is used to indicate plurals eg dog, dogs, and to change the tense of words eg play, plays. 

Most of these letters also make sounds that can be sustained eg Mmmmmm and this too is helpful in helping children blend sounds when they are first learning to read whole words.

Having worked with struggling readers for many years, I know how difficult it can be to find suitable readers for children reading at very low levels. Their teachers and families may only find one or two suitable books from a whole reading series because most readers are written vertically. As a child masters a sound/letter combination or group of letters, he may read the one or two books that are at that level and then can move on to the next level. A child with dyslexia, may need to practise reading at one level many, many times. Therefore, it is planned to create several Reading Mates within the same horizontal level so that a child capable of reading a restricted number of letters, will be able to find several books at the same level within the series.

A sequence of the letters introduced at each level is given below.

CVC books:
Level 1 -  s  a  t  i  m
Level 2 -  n  o  p
Level 3 -  b  c  g  h  k
Level 4 -  d  e  v  f   l
Level 5 -  r  w  j  u  z  qu  y
Level 6 -  ff  ll  ss  zz
Level 7 -  wh  ch  sh  th  ck

VCe books:  
a-e   e-e   i-e   o-e   u-e  mixed