Readers 31-40 introduce the long vowel sounds (e.g. a-e as in cake). This single feature enables children to decode a very large set of new words.
Here the first long vowel digraph is taught. Until this point, all words that contained them were learned as special words.
Words are chosen to reinforce both old and new spelling patterns. Screwdriver, for instance, contains the new sound for Reader 37, ew, as well as er which was first learnt in Reader 14. This repetition helps long term retention.
The long vowel digraphs are reinforced by a careful choice of vocab: e.g. Kate, cake & rake.