The key to understanding the book of Revelation is the recognition of its sequential organization. Most scholars have long accepted the division of the book into three major divisions: the church age (Rev. 1-3), the tribulation (Rev. 4-19) and the post tribulation (Rev. 20-22). Each section occurs in sequential order starting with the church age and ending with the post tribulation. Additionally, the post tribulation section is clearly laid out sequentially starting with the millennial reign and ending with the description of the new heavens and new earth. With historical perspective, the church age is now viewed by most scholars as also being sequential describing the history of the church age its early formative stages through persecution periods, apostasy, worldwide evangelism and ending in apathy. This leaves the tribulation section as the only one that is not viewed as strongly sequential - or does it?
The descriptions in these sections suggest sequential events with a search for one worthy to open the seals on the book, the opening of the seven seals, the blowing of the seven trumpets and pouring of the seven vials. The tribulation period can actually be divided into the rapture, the pre-tribulation scene in heaven, the tribulation period (first 3 and half years), the mid-tribulation chapters which divide the two halfs of the tribulation period, the great tribulation (last 3 and half years), the marriage feast of the lamb and the return of Christ.
The key to the division of the tribulation is the recognition of the chapters 11 and 12 as documenting two events that occur in the middle of the tribulation. Chapter 11 describes the death of the two prophets and lists the deaths as occuring after 3 and half years of ministry. Chapter 12 describes the fleeing of Israel to the place prepared for them. This event is tied to the abomination of desolation that ushers in the great tribulation period. Since both of these events occur at the transition into the great tribulation, it is reasonable to suggest that the events prior to chapter 11 occur in the tribulation period and the events after chapter 12 occur in the great tribulation.