There are many pairs of similar words and phrases in the English language, and mixing them up on a high school or college essay could not only lead to embarrassment, but a lower grade. The word "backup" and the phrase "back up" are one such pair. How do you use "back up" to talk about motion? One way you can use the phrase "back up" is to describe the act of moving backward. For example, you do my homework assignment and might instruct a driver to "back up a few feet, then turn left." A person can "back up," as well. How do you use "back up" when talking about supporting an argument? "Back up" can also describe the act of supporting an assertion or argument. If you make a comment in class, your professor might tell you that you need to back up your comment with textual evidence. How do you use "back up" to describe creating redundant copies? To "back up" an object is the act of creating a redundant copy for use in case the original copy is lost, stolen or otherwise becomes unavailable. This term is often used to describe the act of creating copies of computer files to be stored on another drive or offsite. How do you use the noun "backup?" "Backup" is the nominal form of the verb "to back up" discussed above. The motion usage of "to back up" has no nominal analogue, but the other two do. If your professor asks you to back up your class comments with textual evidence, then the evidence you provide is the backup for your argument. The extra copies of the documents or computer files created in the act of backing them up are called backups. Understanding the correct usage of "back up" and "backup" might be difficult at first, but with practice, you can master these confusingly similar words. If you are still having trouble, print your paper and ask your teacher or professor for assistance. Be sure to create a backup of your work before you do, though.