When you mention the Koran or the Holy Bible in a sentence, do you underline it, just like any other book title? For instance if you wrote, "The Koran and the Holy Bible have longed influenced people around the world" would you underline the Koran and Holy Bible? I have always learned that you don't underline them because they were not written by a specific person and/or they are sacred. Or, maybe you don't underline them because everyone knows what they are? I got in a debate with my teacher over it, and I really want to know that right answer.