The state flag of Maryland was officially adopted in 1904. The flag bears the arms of the Calvert and Crossland families. Calvert was the family name of the Lords Baltimore who founded Maryland, and their colors of gold and black appear in the first and fourth quarters of the flag.

Crossland was the family of the mother of George Calvert, first Lord Baltimore. The red and white Crossland colors, with a cross bottony, appear in the second and third quarters. The flag first was flown on October 11, 1880 in Baltimore at a parade marking the 150th anniversary of the founding of Baltimore. It also was flown October 25, 1888, at the Gettysburg Battlefield for ceremonies dedicating monuments to Maryland regiments of the Army of the Potomac.

Source:

Maryland Symbols - State Flag: Maryland at a Glance

Links:

State Flags - 50 states List
National Flag