The tribulation lasts 7 years and is broken into two 3 ½ year periods (Daniel 9:27). The time period is also referred as the week of Jacob’s trouble.
The last 3 ½ years are referred to as the GREAT Tribulation (Matthew 24:21).

SEALS – TRUMPETS – VIALS (bowls)                (REVELATION)

The first 3 years of Tribulation:

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Seal 1    Anti-Christ – rise to power                          6:1
Seal 2    Wars                                                           6:3-4
Seal 3    Famine                                                       6:5-6
Seal 4    Death                                                          6:7-8

The middle of Tribulation:

Seal 5    A scene from Heaven                                 6:9-11
Seal 6    Events happening on Earth                        6:12-7:17
Seal 7    A short rest Then the TRUMPETS             8:1

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THE GREAT TRIBULATION                               Matthew 24:21
Start of the last 3 years:
                                            8:6

Trumpet 1    1/3 of vegetation                                    8:7
Trumpet 2    1/3 of Sea, Sea life, Ships                     8:8-9
Trumpet 3    1/3 of Fresh Water, many died               8:10-11
Trumpet 4    1/3 Sun, Moon, Stars                             8:12-13
.                 Woe, Woe, Woe,   3 more Trumpets to come:

Near the End of Tribulation:

Trumpet 5    Woe 1, Bottomless Pit Opened            9:1-11
Trumpet 6    Woe 2, Four Angels loosed                  9:13-21
Trumpet 7    Woe 3, 7 Thunders                            10:1-4
.                      Write them not                10:3-4

Revelation 16:1 And I heard a great voice out of the temple saying to the seven angels, Go your ways, and pour out the vials of the wrath of God upon the earth.

Vial 1     Grievous Sores                                          16:2
Vial 2     Sea as blood                                              16:3
Vial 3     Rivers and fountains of waters blood         16:4-7
Vial 4     Sun scorched men                                     16:8-9
Vial 5     Upon the seat of the beast                         16:10-11
Vial 6     River Euphrates dried up                            16:12-16
Vial 7     Great Destructions                                      16:17-21

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Be sure to read the verses for the complete destruction that will happen!