Can the TWF feats be used with a double weapon instead of two weapons?

Question

I was reading another question and answer about a double weapon being used for two weapon fighting and it started me wondering about the wording for something else.

Double:

You can use a double weapon to fight as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting with two weapons, just as if you were using a one-handed weapon and a light weapon. You can choose to wield one end of a double weapon two-handed, but it cannot be used as a double weapon when wielded in this way—only one end of the weapon can be used in any given round.

In TWF combat there is no mention of using double weapons. So the weapon property overrules the more general combat rule.

Now in the TWF feat it seems to still only consider having two weapons, and not having a double weapon.

Improved and Greater TWF only talk about the "off hand" weapon.

So is it reasonable to say that a double weapon is allowed to be used with the two weapon fighting feat chain? It makes me want to build a character who uses TWF with a single weapon thats as large as possible. The cost of upgrading both ends of the weapon will hurt however.

Answer 1

Yes, the feats apply.

So there is a confusion here about two-weapon fighting vs. Two-Weapon Fighting. The former is an option available to all characters, the latter is a feat that provides bonuses (reduced penalties) when using the former. Improved Two-Weapon Fighting and Greater Two-Weapon Fighting are additional feats that expand upon what Two-Weapon Fighting does.

Double weapons give a special exception to the rules for two-weapon fighting (the combat option), allowing you to use these special two-handed weapons for this option instead of the usual pair of weapons. The two ends of the double weapon replace the main and offhand weapons for two-weapon fighting. And when Two-Weapon Fighting (the feat) et al. improve that option, it does not interfere with that replacement, and applies to the two ends of the double weapon just as well as they would have to a pair of weapons.

Note, however, that with the exception of the quarterstaff and in some cases your own racial weapons, double weapons are all exotic, which means you must expend some scarce character resource (usually a feat) to use them. Absolutely none of them are worth a feat ever for anyone, in fact all of them represent taking a feat, an extremely valuable resource, and setting it on fire. Be aware of this before you choose to do so. Furthermore, larger weapon sizes grow in damage extremely slowly, and at least by default the attack penalties you take for using a mis-sized weapon are suicidally punitive. You will miss way, way more than the piddly damage boost is worth. Even things that allow you to ignore those penalties are usually much too expensive to be worthwhile.

Answer 2

Yes, you can use a double weapon with TWF etc.

You can designate which ever head of the double weapon you want as the "main hand" head (probably the one with more enchants). The other head is the "off hand" head.