Before the Version 10 Rulebook (released in 2017), the order in which trigger effects activated was quite different.
SEGOC - Simultaneous Effects Go On Chain:
When 2 or more trigger effects want to activate at the same time, we use "SEGOC" to find out which order they must activate in.
Let's say that each player controls a Dupe Frog (a card with an optional trigger effect), and each player also controls a Sangan (which has a mandatory trigger effect). if "Dark Hole", is activated, the effects will activate as follows: ● CL1: Turn player's (mandatory) Sangan. ● CL2: Other player's (mandatory) Sangan. ● CL3: Turn player's (optional) Dupe Frog.
● CL4: Other player's (optional) Dupe Frog.
As you can see, mandatory triggers always happen first, and the turn player's triggers always happen before the opponent's.
Pre-Ver 10 rulebook, there is an additional layer to this: Trigger order, or "Early Trigger". Effects which "happened first" also need to trigger first. This does not override regular segoc.
EG1: Let's say that Sangan is tributed for Caius the Shadow Monarch. Both of these are mandatory, so using Segoc, you could trigger them in any order. However, since Sangan was sent to the GY first, it must trigger before the Caius. The chain will be: ● CL1: Sangan ● CL2: Caius
EG2: Atlantean Marksman and Mermail Abyssgunde are discarded to summon Megalo. When Megalo is summoned, Marksman and Gunde must trigger before Megalo does, as they were sent to the GY before he was summoned. Furthermore, Marksman must trigger before Gunde, as Marksman is mandatory, and Gunde is optional. The chain will be: ● CL1: Marksman ● CL2: Gunde ● CL3: Megalo EG3: A player controls Inzektor Dragonfly (equipped with Giga-Mantis), and Inzektor Centipede (equipped with Hornet). Hornet is activated, sending itself to the GY, then targeting Giga-Mantis. Since Hornet being sent to the GY was the first thing to happen, the Centipede it was equipped to will have to trigger first. Then, both Dragonfly and Giga-Mantis will trigger, in any order. The chain is:
● CL1: Centipede ● CL2: Dragonfly or Giga-Mantis (you choose) ● CL3: Dragonfly or Giga-Mantis (the other one)
EG4: Dante, Traveler of the Burning Abyss' effect is activated, detaching Cir, and sending Graff. Dante's effect is Since Dante detaches, "then" sends cards to the GY, the chain will be: ● CL1: Cir ● CL2: Graff