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A magic system is a set of rules that magical effects follow in a fictional setting. Magic systems are most elaborate in video games and role-playing games, due to the need to balance the game. A common feature to many magic systems is a way to limit the number of spells a magic user can cast.
The most common way to limit spells is the magic point system. This system gives the magic user a certain number of points (commonly abbreviated MP) that represent either the amount of magic stored in his body or the amount of energy he can channel before being unable to use magic (it is not always specified which). Each spell costs a certain number of magic points to cast. In most systems that use magic points, a magic user has a maximum number of magic points that he can have at any one time, which is different for each magic user. There is almost always a way to restore lost magic points, usually by sleeping or drinking potions.
A few systems that use magic points do not have a maximum number that may be stored, but instead make it more difficult to recover or gain new magic points.
A skill-limited magic system breaks the spells down into a number of skills, usually requiring a die roll to perform. The more difficult the magical effect, the higher the difficulty of the die roll. Such systems are often limited by an increase in the difficulty of the Skill roll based upon the number of spells in a certain time period that have already been cast.
It is common in skill-limited systems for a spellcaster to be able to combine multiple magical skills to perform effects not covered by the skills given. Typically, such combinations are more difficult than the basic uses of the Skills.
A magic system that is limited by a number of spell slots will give a spellcaster a certain number of spells per day that may be cast. These spells may be divided by level, or limited to certain types of spells. When all of a spellcaster's slots are used up, he or she is no longer able to perform magic until steps are taken (usually sleeping and re-studying the spells) to recover the spell slots.
Spell-slot systems often employ a rationale that the spell is forgotten when cast, or that the caster has a finite supply of the ingredients required to cast the spell. In the first case, the spellcaster must re-memorize the spell from a source of such, typically a grimoire. In the second case, the caster must hunt up new ingredients and prepare the equipment needed to cast the spell.
Many magic systems combine features of two or all three of the above. As an example, Mage: The Ascension uses a skill-limited system that may be augmented by spending quintessence to lower the difficulty of a magical skill roll. Rolemaster employs a spell-point system, but includes devices called spell adders, that grant additional spell shots with no associated spell-point cost. Ars Magica uses a skill based system, but a mage can only cast so many spells before becoming too fatigued to continue. Dice & Glory uses a skill-limited system that also limits a magic caster not just by a magical skill roll but also by the source of the caster's magic, a sorcerer uses a point system, an arcane caster can cast a certain number of spells before suffering knockout-point damage causing fatigue or unconscioussness, and casters that channel their magic from other sources are limited to a specific number of spells that they can cast per day. High Adventure Role Playing also uses a hybrid system between the magic point system and the skill system, and to some extent the spell slot version, which requires a skill roll based on the strength of the spell effect limiting the total number of spells cast in a day by a magic cost system with the caster having a certain set of magic points available each day. As in Rolemaster there are item that can reduce the magic point cost for spell as well as item like spell adders that allow extra spells to be cast without the expenditure of magic points.
Warhammer Fantasy Roleplay had a simplistic magic system and Games Workshop long promised a "Realms of Sorcery" book to correct this problem, but they rejected the complete manuscript they received from Ken Rolston.:49 Hogshead Publishing ultimately published Realms of Sorcery (2001), which finally updated the rushed magic system in the Warhammer rulebook.:305
For the magic system of Ars Magica, all magic is based on five techniques and ten forms, and by combining those two elements (e.g., as "Creo Ignem", or Create Fire) a wizard could generate any type of spell.:233
The magic system of Sovereign Stone expanded the game's dice-rolling system; magic could take some time to cast, so each turn a magician accumulated points toward a total until the spell finally went off.:352
The Encyclopaedia Arcane series of alternative magic systems began with Demonology: The Dark Road (2001), Mongoose Publishing's first perfect-bound 64-page book.:394