The events predicted in Revelation take place in a series: a series of sevens repeats three times. This ingenious design communicates a clear message: as each new series cannot begin until the previous series is completed, for each new series is contained within the final event of the previous series, we are given an unmistakable picture of sequential progression (The Design of Revelation, pg. 5).
“The three groups of symbols (Seals, Trumpets, and Bowls) are to be interpreted sequentially. That is, they are to follow from the first to the last one after the other in time. The design of the book confirms this.”
Fred Miller, Revelation: A Panorama of the Gospel Age, Ch. 1, “The Plan and Design of Revelation.”