Addendum

Addendum is an ability word introduced in Ravnica Allegiance for the Azorius Senate.

Description
Spells with Addendum have additional or alternative effects if you cast the spell during your main phase. Addendum is found mostly on instants. The exception is Sentinel's Mark, which is an enchantment.

History
A similar cycle of "main phase matters" instants had previously been featured in Time Spiral.

Rulings

 * Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus. For example, if you cast Sphinx's Insight during your main phase, you gain 2 life during its resolution, after you draw two cards.
 * If a spell with an addendum ability is copied, the copy won't give you the addendum bonus. This is because the copy wasn't cast at all.