If a person cannot write, that person may write an X on any legal document or contract. This X will specify that the signer is aware of what he is entering into. A witness will then sign their name underneath to verify that the contract was not forged or signed without permission. The X is a universally recognized signature for anyone who is unable to write their name. It is otherwise known as making your mark. If you were to challenge the contract in court, you would not be able to point to the fact that an X is on the contract as opposed to your name. Some common reasons why someone couldn't write would include a lack of education or the lack of a physical ability to write. People who lack motor skills would be unable to write their full name in many instances.