Let’s stick to talking about the untap step for now. It has a special property: it’s the only one of all the phases and steps where it’s impossible for players to have priority. Since triggered abilities can’t go onto the stack until a player would have priority, this means that anything triggering during your untap step gets bumped down the line to the next time you have priority: your upkeep step — which is also where your “beginning of your upkeep” abilities trigger. All these triggered abilities are waiting to go on the stack at the same time, so you can order them however you like (it doesn’t matter that one triggered before another, just that they’re going on the stack together).
Suppose it’s the start of your turn, and you’ve got a tapped Arbiter of the Ideal and Thassa, God of the Sea on the battlefield. The Arbiter’s ability triggers when it untaps, and Thassa’s ability triggers a moment later. It’s probably a good play to stack the triggers so that Thassa’s will resolve first. This lets you scry, and if you don’t like the card on top (maybe it’s another Thassa), you can send it to the bottom of your library and get a second shot at manifesting something awesome with your Sphinx.
Today’s Rules Tip written by Jen Wong