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A magic system, which might also be referred to as a magical system, is a set of rules that regulate the magical effects that can be produced in a fictional setting. Magic systems are most elaborate in both video games and role-playing games, because of the necessity to balance the game itself. A common feature of magical systems is either abide by its own environmental or physical law of nature, or use a method of limiting both the quantity and quality of spells that can be cased by a magic user.
Brandon Sanderson has written three articles pertaining to the usage of magic in literature. These are rules Sanderson himself utilizes in writing as guidelines, and may be used for writing in general. Brandon summarized his lecture into 3 key points:
- Sanderson’s First Law of Magic - An author’s ability to solve conflict with magic is DIRECTLY PROPORTIONAL to how well the reader understands said magic.
- Sanderson’s Second Law of Magic - Limitations must be GREATER than powers.
- Sanderson’s Third Law of Magic - Expand what you ALREADY HAVE before you add anything new. 
A magic point is a unit of measure that indicates either or both the amount of magic that can be utilised by a user, and the amount of energy that they can harness to perform magic. Magic points are often abbreviated to MP, and a magic point system is the most common method used to regulate and thus limit the number of spells that can be casted by a magical individual. Such a system provides magic users a specific amount of MP, and each spell causes a specific number of magic points to be consumed upon being casted. Many systems that use magic points assign a magic user 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. Sometimes, even consuming certain kinds of food items may result in the replenishment of magic points.
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. 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.
List of specific examples
- Magic of Dungeons & Dragons consists of spells and magic systems used in the settings of the role-playing game Dungeons & Dragons (D&D). Jack Vance's Dying Earth books provided the model for the magic system of Dungeons & Dragons, with its ideas of magic-users memorizing spells, casting them and forgetting them.:383
- Tunnels & Trolls introduced a magic system full of silly (and humorous) names like "Take That You Fiend!" and "Too Bad Toxin".:35
- 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
- Beyond the Supernatural debuted Palladium's new Potential Psychic Energy (PPE) magic system as well as a system of ley-line based-geomancy.:159
- The second edition of HârnMaster was a simplified version of the game that extracted out the magic systems into Hârnmaster Magic (1997) and Hârnmaster Religion (1998).:183
- Changeling: The Dreaming's original magic system used "Cantrip Cards" which were sold in collectible packs.:218
- 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
- Mongoose's RuneQuest (2006) included a new rune magic system that required characters to quest after physical runes.:399
- Appelcline, Shannon (2013). Designers & Dungeons. Mongoose Publishing. ISBN 9781907702587.