Key Control refers to various methods for making sure that certain keys are only used by authorized people. This is especially important for master key systems with many users. A system of key control needs to include strategies for keeping track of which keys are carried by which people, as well as strategies to prevent people from giving away copies of the keys to unauthorized users. The former may be as simple as assigning someone the job of keeping an up-to-date list on paper. A more complex system may require signatures and/or a monetary deposit. Preventing unauthorized copies typically falls into one of the following five levels.