A Long-sword counts as having finesse when it’s the only weapon wielded by a player. When a player uses two weapon fighting (dual wield) in combination with a long sword, it loses the finesse property.
The logic behind this being as followed:
-When dual wielding long swords (In any dual wielding combination that includes at least one long sword.) strength is a must in order to wave the long sword around (Due to it being longer, and thus heavier when in motion, than other finesse weapons.), resulting in hack and slash type of motions rather than calculated dexterous strikes, thus the finesse is lost.
-When wielding a single long-sword, the player only has one weapon to focus his attention on and is capable of doing so with one or two hands, this allow more fluid, precise and faster combat (Classic Dueling style.), which results in finesse.
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For fair balance, aply the following:
A longsword NEVER counts as having finesse in regards to sneak attack. This is due to the long sword being too... well... long and 'clumsy' to be used subtly and sneakily like say a dagger. Something small and quick like the beforementioned dagger, or a short sword. lends itself much better towards the rogues amongst ye

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Now get me a job at Wizards of the Coast