John Gill Commentary Daniel 6:8

"Now, O king, establish the interdict, and sign the writing, that it be not changed, according to the law of the Medes and Persians, which altereth not." — Daniel 6:8 (ASV)

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 alters 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) .