And a stone was brought, and laid upon the mouth of the den - Probably a large flat stone sufficient to cover the mouth of the cave, and so heavy that Daniel could not remove it from within and escape. It was usual then, as it is now, to close up the entrance to sepulchres with a large stone. See John 11:38; Matthew 27:60. It would be natural to endeavor to secure this vault or den in the same way - on the one hand so that Daniel could not escape from within, and on the other so that none of his friends could come and rescue him from without.
And the king sealed it with his own signet - With his own seal. That is, he affixed to the stone, probably by means of clay or wax, his seal in such a way that it could not be removed by anyone without breaking it, and consequently without the perpetration of a crime of the highest kind - for no greater offence could be committed against his authority than thus to break his seal, and there could be no greater security that the stone would not be removed. On the manner of sealing a stone in such circumstances, compare the note at Matthew 27:66.
And with the signet of his lords - That it might have all the security which there could be. Perhaps this was at the suggestion of his lords, and the design, on their part, may have been so to guard the den that the king should not release Daniel.
That the purpose might not be changed concerning Daniel - By the king. Probably they feared that if there was not this security, the king might release him; but they presumed that he would not violate the seal of the great officers of the realm. It would seem that some sort of concurrence between the king and his nobles was required in making and executing the laws.