The first extant Estonian book is a bilingual German-Estonian translation of the Lutheran catechism by S. Wanradt and J. Koell dating to 1535, during the Protestant Reformation period. An Estonian grammar book to be used by priests was printed in German in 1637. The New Testament was translated into southern Estonian in 1686 (northern Estonian, 1715). The two languages were united based on northern Estonian by Anton thor Helle.