Penalties

Various effects can cause you to take penalties to ability scores, Armor Class, die rolls, movement, or other statistics. Unlike bonuses, penalties do not have types. However, multiple applications of the same penalty generally don’t stack—only the highest penalty applies.

For example, suppose the GM assigns a penalty to Acrobatics checks on a frozen lake because its surface is slippery, and one of the PCs’ enemies has a device that applies penalties to Acrobatics checks in an area by freezing the ground so that it is icy and slippery. Causing the already-frozen lake to again be frozen doesn’t make it any more slippery and in fact doesn’t change it much at all. Therefore, only the highest of these two penalties applies to Acrobatics checks creatures attempt on the doubly frozen ice.

The GM serves as the ultimate arbiter of whether or not two situations that apply penalties count as the same source and thus whether the penalties stack. A GM might decide that while freezing a lake twice doesn’t make it any more slippery, pouring oil on its surface does, and additional penalties might accrue from other environmental factors, such as a strong wind, an earthquake, or the lake’s surface having frozen unevenly into a series of inclines and depressions.