Finally, although most ESL course books are well organized with many different kinds of activities, however, they do not provide enough details in other aspects of language study. A good example would be in the study of grammar. The grammar section in each unit of the course book usually does not provide enough explanation or practice questions. Relying on the course book to provide the students with adequate knowledge of grammar would not be enough, especially when a teacher spends between two to three weeks to cover a single unit of the course book. This means that in a typical semester, students only receive between four to six different types of grammar instruction, a number far behind from what they could have been studying if the students had a grammar textbook where they can study a different unit every week with lots of practice activities.