You can get a host of benefits from wielding a twohanded
weapon, such as 1-1/2 times your Strength bonus on
damage (and twice your damage bonus from the Power
Attack feat) and a +4 bonus on your opposed attack roll if
someone tries to disarm you. So when is a weapon “twohanded?”
For example, a lance is a two-handed weapon,
right? But you can wield it in one hand when you’re
mounted. Since the weapons table shows that a lance is a
two-handed weapon, I get all the two-handed benefits no
matter how I wield the lance, right?
Wrong. Table 7–5 in the Player’s Handbook lists weapons
as light, one-handed, or two-handed strictly as a matter of
convenience. These size categories are always relative to the
wielder’s size, as explained in some detail in the section on
weapon size on page 113 in the Player’s Handbook (also see
next question).
When the combat rules speak of “two-handed” weapons,
they’re referring to how the weapon is being used. A Medium
character using a Medium longsword in two hands is using a
“two-handed” weapon. The same character using a Medium
lance in one hand while mounted is using a one-handed
weapon. Light weapons are an exception. If you wield a light
weapon in two hands you get no advantage on damage (see
page 113 in the Player’s Handbook). Likewise, you always
take a –4 penalty on your opposed roll when you’re wielding a
light weapon in a disarm attempt (when someone tries to
disarm you or you try to disarm someone) regardless of
whether you wield it one- or two-handed.