The Merchant Mariner's Document (MMD), previously called a Z-Card, is a kind of Merchant Mariner Credential previously issued by the United States Coast Guard in accordance with the STCW guidelines, and, until completely phased out, remains one of the standard documents required for all crewmembers of U.S. ships with a Gross Tonnage of over 100.
The document is about the size of a passport, and contains the sailor's information regarding date of birth, the location of issue, nationality, and the shipboard duties he or she is qualified for.
Today, the document still serves this purpose, and is regarded as a proof of identity and a passport when a sailor is in a foreign country.
Prior to the early 1990s, Z-Cards were obtained free of charge and were good for life, but increased safety standards in the maritime industry sought to require all active mariners to renew these documents and constantly train to stay abreast of any advancements in their field.
All applicants for a Z-Card are required to apply, take a drug test, and wait for a criminal background check to complete before receiving their documents, which can take from a few weeks to several years.