Regular and irregular words
Regular words are those in which every letter represents its most common sound. For instance kin is regular, but kind is not: the most common sound for i is the sound it makes in sit.
Irregular words are those in which one or more letters do not represent their most common sounds.
However, a student who knows the most common sounds not just for individual letters but for letter combinations as well, such as sh and ee, will be able to sound out words, such as ship and free, even though these words are, strictly, irregular. In beginning reading it's helpful to distinguish irregular words that could still be sounded out in this way from truly irregular words, such as eye and they. In FreeReading, we introduce a subcategory of irregular: regularish.
Regularish words are irregular words that can be sounded out correctly by a student who knows the most common sounds of the letter combinations (listed here). For instance, moon is regularish but door is not.
Note that high-frequency words, meaning words that occur frequently in English, include both irregular words (the, was) and regular words (can, not).