Teachers use many different criteria when selecting works to assign to a class. For second language learners, two important criteria are that the vocabulary and syntax are appropriate to the students' abilities. Most teachers also want to assign works that are historically or culturally important, formally innovative or exemplary, and thematically related to the proposes of the class. Another increasingly important factor in selecting works is that most parts of the literary canon are so widely written about that students may be able to read commentaries on the work as opposed to reading the original themselves. Thus, teachers are increasingly assigning obscure works in the hope of ensuring that students must actually do the assigned reading to pass the course.