Daniel 6:8

Now, O king, establish the decree, and sign the writing
For they had not only agreed upon it among themselves what to propose, as to the substance of it; but they had drawn it up in writing, ready to be signed, which they urge to have done immediately: that it be not changed, according to the law of the Medes and Persians,
which altereth not;
when once signed by the king: mention being made of both the Medes and Persians, shows that these two nations were now united in one government; that Darius and Cyrus were partners in the empire; and it is easy to account for it why the Medes are mentioned first; because Darius was the Mede, and Cyrus the Persian; the one the uncle, the other the nephew; but afterwards, when a Persian only was on the throne, then the Persian is mentioned first, ( Esther 1:19 ) .