CK-12 FlexBooks are the open textbooks designed for United States K-12 courses. CK-12 FlexBooks are designed to facilitate conformance to national and United States and individual state textbook standards. CK-12 FlexBooks are licensed under a Creative Commons BY-NC-SA license. CK-12 FlexBooks are free to use online and offer formats suitable for use on portable personal reading devices and computers - both online and offline. Formats for both iPad and Kindle are offered. School districts may select a title as is or customize the open textbook to meet local instructional standards. The file may be then accessed electronically or printed using any print on demand service without paying a royalty, saving 80% or more when compared to traditional textbook options. An example print on demand open textbook title, "College Algebra" by Stitz & Zeager through Lulu is 608 pages, royalty free, and costs about $20 ordered one at a time (March, 2011). "(Any print on demand service could be used - this is just an example. School districts could easily negotiate even lower prices for bulk purchases to be printed in their own communities.)" Teacher's editions are available for educators and parents. Titles have been authored by various individuals and organizations and are vetted for quality prior to inclusion in the CK-12 catalog. An effort is underway to map state educational standards correlations. Stanford University provided a number of titles in use.