Subtle (3.5e Equipment)
From D&D Wiki
Subtle: Any weapon with the subtle enhancement has a doubled critical multiplier, not range. For example, a +1 subtle dagger goes from a critical of of 19-20/x2 to 19-20/x4. Ranged weapons bestow this power on their ammo.
Moderate evocation;CL 15; Craft Magic Arms and Armor, keen edge; Market Price: +2 bonus