The Broken Hourglass - Principles of Magic @ Official Site
Planewalker has written up quite a bit of detail on the magic system for The Broken Hourglass, which features some intriguing mechanics. Here's a snip:
With these explanations in mind, it becomes easier to understand that our magic system is a mana-potential system. The amount of mana possessed by each character represents how much magical energy they can divide up among one or many tasks at a given time. Any effect which has a long duration is considered a standing enchantment, and standing enchantments tie up mana until they are released. A mage with 15 mana points who invests five mana points in a damage-protection spell is reduced to having 10 mana left to spend. When he releases the armor spell, he will once again have 15 mana available. Beneficial standing enchantments typically remain in play until party members choose to drop them. Deleterious standing enchantments, such as an attribute penalty or a spell to lower resistance to damage, typically remain in play until the end of combat, or until the party rests.
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