According to the manual you get them at certain levels:
Fighter, Barbarian: Level 20
Ranger, Paladin, Mage, Sorceror: Level 18
Cleric, Monk: Level 22
Druid: Level 15
Thief, Bard: Level 24
I believe you have to reach that level in whatever class to use these skills, with singles you would get more. As with everything, multi-classes would lose out some but I really don't know to what degree.
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Can an elf get a break anywhere these days?
http://angelfire.com/rpg/aurican