The Maryland State Flag consists of the arms of the Calvert family quartered with the arms of the Crossland family. it was officially adopted by law in 1904. Maryland has the distinction of having the only flag in the United States that is based on heraldic emblems.

The father of George Calvert, first baronet of Baltimore and founder of the colony of Maryland, was Leonard Calvert, a country gentleman of Yorkshire. He married Alicia Crossland, daughter of John Crossland, another Yorkshire gentleman. Both families were of the class entitled to have arms. Because Alicia Crossland had no brother and so was the heiress of her family estate, she was permitted under heraldic law to quarter her arms with those of her husband.

The Calvert and Crossland arms are first shown quartered together in the Seal of Maryland. The original Seal probably had the quartering, but unfortunately it was lost and there is no exact description of the design. It was replaced by Cecil Calvert, the second Lord Proprietary in 1648. He remarked that the replacement was very like the first. The quartering of the two coats of arms together, therefore is of old usage in the Seal. However, it does not appear in the Flag until much later.

From 1634 until the American Revolutionary War, there was no definite state flag in existence, either in custom or in law. There were a number of variations of the design, used pretty much ad libitum, for decorations and military emblems. In 1885, Clayton C. Hall, scholar of Baltimore, delivered an address on the Maryland Seal to the Maryland Historical Society, subsequently having the speech reprinted in pamphlet form. He gave a copy of the pamphlet to Mr. Sisco, a flagmaker of Baltimore the following year. Mr. Sisco manufactured a number of flags in the pattern of the one used today, apparently basing his design on information about the Seal gathered from the work of Mr. Hall.

The present flag, adopted by law in 1904, is composed of both family arms quartered and then reversed. That is, reading horizontally from the top of the staff, the first and second quarters are the Calvert and Crossland arms respectively. Below are the same arms in reverse order.

The Calvert arms are six pales (perpendicular stripes) alternately or (gold) and sable (black) transverses from dexter chief (upper right corner) to sinister base (lower left corner) by a bend (band probably representing an ancient shoulder belt for carrying a sword) counterchanged (black where is crosses gold stripe, gold where it crosses a black one). In the upper quarter, the first gold pale must be next to the flagstaff, and the extreme upper corner of the bend will be black. This is important, because if the flag is displayed upside down, a black pale will be next to the flagstaff, which is incorrect.

The Crossland arms are quarterly argent (silver) and gules (red) and cross bottony counter-changed. In flags the color silver is represented by white. The cross bottony is a cross with extremities resembling a tre-foil plant.