Curriculum Description (from publisher)
Preliteracy primers which introduce all consonant letters; they are designed to be used before the Explode the Code series. Covering early literacy skills and letter formation, students will learn through visual, auditory, and kinesthetic activities that include tracing, matching, tracking, copying, and following directions.
Each of our kids used Explode the Code, starting before they could read. We started each of them with the Primer (A, B and C) books and then moved onto the others.
UPDATE 2018: We used Explode the Code primers with all but our youngest daughter.