Simultaneous effects: the current turn’s controller chooses order

SRD 5.2.1 · Turns

At the table

1. Identify the exact timing stated by each effect.
2. If two or more happen at that same time on a turn, identify whose turn it is.
3. That player or the GM chooses the order; then resolve the effects in that order using their own text.

Example decision

Two effects both occur at the start of a player character’s turn. One originated from a monster. Who chooses their order under the general rule?

The player whose character’s turn it is.

The deciding person is tied to whose turn it is, not to who created an effect. At the start of a player character’s turn, that player chooses the order of the simultaneous effects. The choice of order does not rewrite either effect’s prerequisites, amount or duration.

Watch for this mistake

Effects at the start and end of a turn are not simultaneous simply because they occur within one turn. Compare their actual timing before offering an ordering choice.

Ask the table

Do the effects share the same timing point, and whose turn contains that point?

Source rule

If two or more things happen at the same time on a turn, the person at the game table—player or GM— whose turn it is decides the order in which those things happen. For example, if two effects occur at the start of a player character’s turn, the player decides which of the effects happens first.

Source: SRD 5.2.1, 187. Online guide.