The current flag of Alabama (the second in Alabama state history) was adopted by Act 383 of the Alabama Legislature on February 16, 1895:
"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." – (Code 1896, §3751; Code 1907, §2058; Code 1923, §2995; Code 1940, T. 55, §5.)
The cross of St. Andrew referred to in the law is a diagonal cross, known in vexillology as a saltire.
Text modified from "Flag of Alabama" on Wikipedia which is licensed under CC BY-SA 4.0.