"The flag of the State of Alabama shall be a crimson cross of St. Andrew on a field of white. The bars forming the cross shall be not less than six inches broad, and must extend diagonally across the flag from side to side." A cross of St. Andrew is a diagonal cross, known in vexillology as a saltire. Because the bars must be at least six inches (15.24 cm) wide, small representations of the Alabama flag do not meet the legal definition and can not legally be considered "flags" of Alabama.

It is commonly believed that the crimson saltire of the Flag of Alabama was designed to resemble the blue saltire of the Confederate Battle Flag. The Battle Flag was square-shaped, and Alabama's flag is sometimes shown as a square. However, although the legislature did not specify the proportions, a "cross of St. Andrew" is understood to be rectangular. The authors of a 1917 article in National Geographic expressed their opinion that because the Alabama flag was based on the Battle Flag, it should be square. In 1987, the office of Alabama Attorney General Don Siegelman issued an opinion in which the Battle Flag derivation is repeated, but concluded that the proper shape is rectangular, as it had been depicted numerous times in official publications and reproductions.

The saltire design of the Alabama state flag also bears resemblance to several other flags. It is identical to the flag of Saint Patrick, incorporated into the Union Flag of the United Kingdom to represent the union of the Kingdom of Great Britain with the Kingdom of Ireland. However, it likely owes its origin as a simplification of the Cross of Burgundy Flag used by the Spanish in the New Spain and as the basis of military flags such as that of the Regimiento de Infanteria de Luisiana which took part in the Battle of Mobile as part of the Gulf Coast campaign of the American Revolution.