Most of World of Warcraft’s stacking buffs are multiplicative. If you have one buff that increases an attribute by 10%, and another that increases that attribute by 15%, and those buffs stack, the final attribute A will be A * 1.1 * 1.15, or A * 1.265 (a total 26.5% increase).

However, the Ribbon Dance buff, which increases XP gained from killing monsters by 10%, and heirloom shoulders, which increase XP gained from killing monsters and completing quests by 10%, are additive. Instead of XP * 1.1 * 1.1, they grant XP + (XP * 0.1) + (XP * 0.1).

Fortunately, this costs you a very small amount of mob-kill XP – the final number is XP * 1.2 instead of XP * 1.21 – but it’s worth noting if the XP isn’t quite racking up like you thought it would.