Guild Wars 2

When you have a damage bonus from an item or trait e.g. Sigil of Force, Sigil of Night, do they add up?

So If I run both sigils does that mean I have 15% more damage? Or how does this work?

And what if I have a trait in addition to the damage: For example If I am running Executioner on a Thief, that adds 20% damage below a certain threshold. Will that mean that if I have the Sigil of Force and Sigil of Night I total out to an additional 35% more damage?

Not sure about Force and Night stacking, but everything else does so traits+sigils all stack same with any consumables your using.
Slaying sigils also stack with Force/Night.