It only says: "The flag of the United States shall have thirteen horizontal stripes, alternate red and white, and a union consisting of white stars on a field of blue."
Then goes on to give proportions of everything.
I suppose the key may be the word "alternate". Some dictionaries define it with some sense of order to its operands. The Collaborative International Dictionary of English says "To perform by turns, or in succession". WordNet says "occurring by turns; first one and then the other;".