Quartz
Hero
Subject really asks it all, but just to elaborate, suppose no magic spells or effects stack on the d20 roll at all? If you have a +3 sword and Guidance in effect, they don't stack (so you better roll a 4). If you have +1 armour and +1 shield, they don't stack. If you have a +3 sword and a Belt of Giant Strength, they don't stack.