An identity document (also called a piece of identification or ID, or colloquially as papers) is any document that may be used to prove a person's identity. If issued in a small, standard credit card size form, it is usually called an identity card (IC, ID card, citizen card), or passport card. Some countries issue formal identity documents, as national identification cards which may be compulsory or non-compulsory, while others may require identity verification using regional identification or informal documents. When the identity document incorporates a person's photograph, it may be called photo ID.