The Blackline Masters are folders of supplementary practice sheets that are copyright-free. There is one folder of Blackline Masters for each of the Books A-G (K-6th Grade). Only one folder of Blackline Masters need be purchased for each book, and as many copies as needed may be reproduced from it. The practice sheets are not duplicate copies of pages in the books, but they are designed to complement the lessons in the books. Each practice sheet references the page number in the book of that particular lesson, making it easy to integrate the lessons in the book with the extra practice opportunity offered in the Blackline Masters.