The rules distinguish between "what you're trained to fight with" and "how you're trained to fight with it".
What Weapons You Fight With (Weapon Training)
What you're trained to fight with are specific weapons or weapon groups. For example, "any missile weapons and any one-handed melee weapons" (thief) or "daggers, swords, spears, and polearms" (bladedancer).
For these purposes, a weapon's "handedness" is defined by the minimum number of hands required to use it. A shortsword and sword are therefore both one-handed weapons. A two-handed sword is a two-handed weapon. All two-handed weapons are noted as such in parantheses in the Weapons and Equipment table of the rules.
How You Fight With Your Weapons (Fighting Style)
The rules restrict access to three important fighting styles: Weapon and Shield; Weapon With Two Hands; Weapon in Each Hand. Essentially, it's a question of what you do with your off-hand. Do you wield a shield? Do you wield a second weapon? Or do you add it to the grip on your main weapon for added strength?
Some weapons are incompatible with particular styles.
- Tiny melee weapons (daggers) and small melee weapons (hand axes, short swords) cannot be used in "Weapon With Two Hands" style.
- Large melee weapons (e.g. polearms, two-handed swords) cannot be used in "Weapon and Shield" and "Weapon in Each Hand" style.
- Medium melee weapons (e.g. sword, battle axe, mace, flail, spear) can be used in any fighting style
The Interplay of Weapon and Fighting Style
The ability to use a particular weapon does not convey the ability to fight in a particular style. For example, if you can use a battle axe, but can't fight in "weapon with two hands" style, then you cannot wield a battle axe with two hands.
The *inability* to use a particular weapon does not remove the ability to fightin a particular style. For example, consider the case of an explorer, who can fight with a weapon in two hands, but cannot use Two-Handed Weapons. What this means is that an explorer can wield a spear with two hands, or a battle axe with two hands, but cannot wield a two-handed sword or polearm.
Rules As Intended
In re-reading the core rulebook, I note a few areas where the rules-as-written diverged from the rules-as-intended (or at least I am not as explicit about an implicit intent). Specifically:
1. Thieves are allowed to fight with two hands on a weapon. (If you build a thief using custom class rules, you'll note that the thief does not make any fighting style trade-off, so he's eligible for two fighting styles.)
2. Dwarven Vaultguards are permitted to fight with dwarf-sized two-handed weapons such as dwarven great axes and morning stars. These would be weapons with a shorter haft but retaining a heavy striking head requiring two hands to use. The clumsy language "human sized two-handed weapons" was very awkward. The intent is to restrict dwarves from using ill-sized weapons, not to reduce their striking power per se. No Dwarven class is permitted to fight with human-sized two-handed weapons.