During the pre-marriage licensing period, 1908 - 1951, we seek the Certificate first, because it has a definite date. Once the Certificate is found, then we seek the License. The License was applied for at the county level in New York. Potential brides and grooms had to appear in person at the county where the bride lived, no matter where the actual marriage ceremony was to take place. No photo ID was required then. In NYC couples often slipped away from work at lunch time to apply. Note the mode of dress in Norman Rockwell's painting "Marriage License." http://www.allposters.com/-sp/Marriage-License-June-11-1955-Posters_i7553226_.htm At one time blood testing wad required to apply for the License, probably due to disease that wartime soldier might carry. A marriage LICENSE was valid for a given period of time. They are filed by county first, then by the date of application. Researchers know to look backwards from the date of marriage ceremony in the county where the bride resided. Can you imagine the number of Marriage Licenses applied for in one month in Manhattan? This is why searching for marriage licenses is more time consuming than the usual certificate. Marriage Licenses are important because the bride and groom provided the facts, not a third party.