Main Skill - The user can learn E-S rank jutsu for this skill.
Second Main Skill - The user can learn E-S rank jutsu for this skill.
Secondary Skill - The user can learn E-A rank jutsu for this skill.
Tertiary Skill - The user can learn E-C rank jutsu for this skill.

Main Skill - Chosen at Genin rank
Second Main Skill - Chosen at Chuunin rank - Requires a week of training, 7 paragraphs each day to learn
Second Skill - Chosen at Chuunin rank - Requires a week of training, 7 paragraphs each day to learn
Tertiary Skill - Chosen at Jounin rank - Requires a week of training, 7 paragraphs each day to learn

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If you want to learn a jutsu of a higher rank than your skill reaches, you are allowed to sacrifice a whole other rank for that one higher ranked jutsu.

Ex. Your Tertiary skill is Ninjutsu. The highest ranked jutsu you can learn is C. If you want to learn one B rank, you need to sacrifice a different rank. Either E, D or C. Once done you can't learn any Ninjutsu of that rank ever again, and you cannot sacrifice the rank if you've already learned a Ninjutsu of that rank.

If you want to jump two ranks above your skill limit, it costs two whole ranks.

You may not learn an S rank from any skill other than your two Main Skills.