The way in which triggered abilities have been dealt with has changed
   dramatically throughout the history of the game.

   When any instruction is followed in the game, it may match the trigger
   event of any number of triggered abilities.  Each time this happens,
   the triggered event triggers.  When a player is in the process of
   gaining priority to play spells and abilities, players put any triggered
   abilities that have triggered onto the stack.  First the active player
   adds all the triggered abilities that he or she controls, in the
   order of his or her choice, and then the non-active player does so.
   With multiple non-active players, players go in turn order.

   With this system, triggered abilities never go onto the stack while
   another spell or ability is resolving.  They always wait until a
   player is about to gain priority.  However, the order of events within
   the resolution of the spell or ability does still matter.  For example,
   during the resolution of Hypergenesis, a Where Ancients Tread enters
   the battlefield, and then its controller also puts a Woolly Thoctar
   onto the battlefield:

   Hypergenesis    Sorcery
/ Hypergenesis is green.
/ Suspend 3 - {1}{G}{G} (Rather than cast this card from your hand, pay
   {1}{G}{G} and exile it with three time counters on it. At the beginning
   of your upkeep, remove a time counter. When the last is removed, cast
   it without paying its mana cost.)
/ Starting with you, each player may put an artifact, creature, enchantment
   or land card from his or her hand onto the battlefield. Repeat this
   process until no one puts a card onto the battlefield.

   Where Ancients Tread  {4}{R}  Enchantment
/ Whenever a creature with power 5 or greater enters the battlefield
   under your control, you may have Where Ancients Tread deal 5 damage
   to target creature or player.

   Woolly Thoctar  {R}{G}{W}  Creature - Beast  5/4

   The Where Ancients Tread "sees" the Woolly Thoctar, and triggers.  After
   the Hypergenesis has finished resolving, the controller of the Where
   Ancients Tread puts its triggered ability on the stack, and chooses a
   target for it. This could target another creature that was put onto
   the battlefield after the Woolly Thoctar.

   Some damage prevention effects have "side-effects", extra things that
   are written along with the damage prevention:

   Brace for Impact  {4}{W}  Instant
/ Prevent all damage that would be dealt to target multicolored creature
   this turn. For each 1 damage prevented this way, put a +1/+1 counter
   on that creature.

   These are not usually triggered abilities.  They just happen as part
   of the event that would have dealt the damage.