A source of instruction is required to learn a category skill. Instruction speeds the learning of teachable subskills. If instruction in the category skill for a teachable subskill is available, then the student will learn the skill in half the normal time. If instruction in the specific subskill is available, then the student will learn the skill in one-quarter the normal time. If a noble studies this skill without any instruction, it will take him two weeks of study to learn the skill.
Instruction has no effect for unteachable subskills or guild skills.
In some cases, a noble may have more than one source of instruction available. When a book and some other source offer the same type of instruction, then the other source is selected. The benefits of instruction are calculated daily. If instruction ends prematurely for a category skill that requires instruction, then the noble's study command also ends.