Any skill imaginable is available as a Secondary Ability. The XP cost is the same as for Primary Abilities.
A character using a Secondary Ability to perform an action gains a -1 Difficulty adjustment (or a -2 Difficulty adjustment for a non-combat skill). This is cumulative with the adjustment from Ability Aptitude or Ability Mastery and any adjustment from a maneuver. The dice pool used is that of the appropriate Secondary Ability, not that of the Primary Ability.
Some of the more common Secondary Abilities include Business, Cosmology, Lore, and Martial Arts. There is no need to list or detail most Secondary Abilities, since there are an infinite number of them, but the last three are worth expanding on.
Cosmology, a Secondary Ability of Occult, is a rare knowledge concerning the "Worlds other than these." Where do ghosts come from? Or Faeries? Or Spider Aliens? What is this "Bleed" the Harbingers mutter about?
A starting character may not begin with Cosmology unless the character's backstory justifies it. Even then, a starting character may not begin with more than three ranks.
Lore is a Secondary Ability of Occult which offers precise answers. There are an infinite number of Lores: Bygone Lore (mythological creatures), Psychic Lore (psychic powers), Vampire Lore, Werewolf Lore, Mythos Lore (Cthulhu Mythos), Wyrm Lore … all separate skills.
A starting character may not begin with Lore unless the character's backstory justifies it. Even then, a starting character may not begin with more than three ranks.
Mythos Lore: The character's potential Morality will be 10 - Mythos Lore.
Wyrm Lore: When a rank of Wyrm Lore is acquired, the character must make a Conviction check at a Difficulty of the new rank + 3.
Hard and Soft Martial Arts are Secondary Abilities of Brawl. Hard Martial Arts (karate, taekwondo) and Soft Martial Arts (judo, aikido) are separate skills. Kicks and punches would be considered Hard Martial Arts attacks; sweeps and throws would be considered Soft Martial Arts attacks.
Here is a Secondary Abilities List it is not complete of course, because there are no limits to the possibilities, these are some of the more commonly used ones however.
