Three sets of seven judgments (the seals, trumpets, and bowls) are central to Revelation. Some believe these judgments happen in a sequence, one after another, totaling twenty-one judgments.
However, it is more likely that the relationship is cyclical, similar to other Jewish apocalyptic writings (compare Daniel 2, 7, 8, 11). Each set of judgments becomes more intense and adds new details about God's judgment on those who rebel against him. In this view, all three cycles end at the same time, with the return of Christ.