Set a mark upon the foreheads, etc. The command reminds us of that given to the destroying angel in Exodus 12:13, and has its earlier and later analogues in the mark set upon Cain (Genesis 4:15), and in the "sealing" of the servants of God in Revelation 7:3.
Here, as in the last example, the mark is set, not on the lintels of the doorposts, but upon the "foreheads" of the men. And the mark is the letter tau, in old Hebrew, that of a cross +, and like the "mark" of mediaeval and (in the case of the illiterate) of modern usage, seems to have been used as a signature, and is rightly so translated in the Revised Version of Job 31:35.