Prerequisites

Prerequisites are discussed on B169 and apply to skills in general:
:To study the advanced skill, you must have at least one point in the prerequisite skill.
It also includes a note for non-skills:
:A few skills have advantages as prerequisites.
:In order to learn such a skill, you must possess the required advantage.
:If you do not have the advantage, and cannot acquire it in play, you can never learn that skill.
==Magic==
B77 is important here:
:clerical spells do not have prerequisites
Due to this, any penalties for lacking them would likely not apply.
Page 6 of GURPS Magic reiterates how they apply to spells:
:requirements you must meet in order to learn the spell.
:If the prerequisite is another spell, you must have at least one point in the prerequisite spell before you can study the advanced spell.
:Not all prerequisites are spells.
:Some spells require a minimum Magery level
:..
:A few spells require a minimum basic attribute score, an advantage, or even a mundane skill.
M200 covers Alternate Prerequisites and talks about how Ritual Magic techniques derive penalties from the Prerequisite Count
M201 has a Shortening Prerequisite Chains option
==Thaumatology==
specifically does comment on T56 regarding Modular Abilities:
:The GM could require the spellcaster to memorize all of an advanced spell’s prerequisites before he memorizes the spell itself,
:but that’s rather restrictive.
:It’s usually enough to require him to meet any non-spell prerequisites (Magery, IQ minima, etc.) and
to have the prerequisite spells in his grimoire.
as well as Wild Talent:
:A wizard who can only use spells that he has studied in the past, in enough detail to transcribe notes and descriptions to his personal grimoire, has a further -40% Accessibility limitation.
==Using skills WITHOUT prereqs==
At least two known sets of rules exist for this:
===Wild Talent===
if you lack any of the skill’s prerequisites, your skill is at -4 until you acquire them,
and you cannot improve the skill in the interimBasic Set pg 99
===Forgetfulness===
If (for instance) a forgotten spell is a prerequisite for other spells, the other spells can still be used, at -2 to skill.Basic Set pg 250; GURPS Magic pg 135
===other situations===
There are other instances in which someone could learn an advanced spell (first knowing prereqs) perhaps without knowing prereqs. Whether these cause penalties (and if so, WHICH) may be unclear
Those include:
====crit fails====
B236/M7:
:Spell fails entirely. Caster temporarily forgets the spell.
====Modular Abilities====
There is no mention as to whether prereqs are necessary here.
====not Maintaining Skills====
if using B294's optional rule, failing an IQ roll for a prereq not used for 6 months could result in losing a point spent in it:
:If a skill with only one point in
it degrades, it drops to default level
:(that is, you are no better than someone without training)
:and cannot degrade further.
====magic borrowing====
M105's Memorize makes reference to this:
:cannot be used to learn a skill or to retain a magically borrowed one.
This likely refers to Lend Skill and Borrow Skill from M47. Note however that this says "Does not
work on languages or spells."
==modifiers==
*No Spell Prerequisites on T67 for designing Power Investiture as modified magery
ja:前提条件
==see also==
*Missing Prerequisites
*Unusable Prerequisites aka "useless prerequisites"
Category:Rules