In reply to your 4/28 post Is that possible, or is there another explanation? (fivedoves.com) in my opinion I don't think that the book of Revelation is written entirely in chronological order.  I think the martyrs seen under the altar in Rev. 6:9 and 6:10 when the 5th seal is opened are just those who are martyred for not worshipping the Antichrist nor taking his mark in the forehead or in the hand in order to buy or sell, and they can be seen in heaven in Rev. 7:9-17 after the 6th seal is opened which indicates that they were slain during the 1,260 day reign of the Antichrist after which the Lord Jesus Christ unleashes His wrath on Earth. If that is the case, then Revelation chapter 13 about taking the mark of the beast would have to take place between the 5th and 6th seal.