Article 7.3 establishes the procedure that SSA will follow in converting periods of coverage under the Danish system and periods of Danish residence as defined in Article 7.2 into equivalent periods under the U.S. system. Periods of coverage under the U.S. system are measured in terms of calendar quarters while Danish periods of contribution and periods of residence are measured in months. Beginning in 1978, U.S. quarters of coverage are based on the amount of a person's annual earnings (e.g., for 2007, $1,000 in earnings equals one quarter of coverage). Under Article 7.3, SSA will credit one quarter of coverage in a calendar year for every 3 months of contribution or residence during which a person worked certified for that year by the Danish agency. However, SSA will not credit months of Danish contribution or residence which fall within a calendar quarter which has already been credited as a U.S. quarter of coverage. In addition, SSA will not credit more than 4 quarters of coverage for any calendar year.